Importance of Stencil for PCB Assembly

The surface mount assembly process uses a stencil as a gateway to an accurate and repeatable solder paste deposition. A stencil is a thin sheet or foil of brass or stainless steel with a circuit pattern cut into it, matching the positional pattern of surface mount devices (SMD) on the printed circuit board (PCB) for which the stencil is to be used. After accurately positioning and matching the stencil over the PCB, a metal squeegee forces solder paste through the apertures of the stencil to form deposits on the PCB for holding SMDs in place. The solder paste deposits, when passed through the reflow oven, melt and secure the SMDs to the PCB.
The design of the stencil, especially its composition and thickness and the shape and size of its apertures, determines the size, shape, and positioning of the solder paste deposits and this is crucial to ensuring a high-yield assembly process. For instance, the foil thickness and the aperture opening size define the volume of paste deposited on the board. An excess of solder paste causes balling, bridging, and tomb-stoning. Low amounts of solder paste cause dry solder joints. Both compromise the electrical functionality of the board.
Optimum Foil Thickness
The types of SMDs on the board define the optimum foil thickness. For instance, component packages such as 0603 or SOICs with a 0.020” pitch require rather thin solder paste stencils, whereas a thicker stencil is more suitable for components such as 1206 or SOICs of 0.050” pitch. Although stencil thickness for solder paste deposition ranges from 0.001” to 0.030”, the typical foil thickness that a majority of boards use ranges from 0.004” to 0.007”.
Technologies for Making Stencils
At present, the industry uses five technologies for making stencils—laser-cut, electroformed, chemically etched, and hybrid. While the hybrid technology is a combination of chemical etching and laser-cutting, chemical etching is very useful for making step stencils and hybrid stencils.
Chemical Etching for Stencils
Chemical milling etches metal masks and flexible metal mask stencils from both sides. As this etches not only in the vertical direction but also laterally, it causes undercutting, and makes the openings larger than desired. As the etching proceeds from two sides, the tapering on the straight wall causes the formation of an hourglass shape, leading to extra deposits of solder.
As etching the stencil openings does not produce a smooth result, the industry uses two methods for smoothening the walls. One of them is electropolishing, a microetchng process, and the other is nickel plating.
Although a smooth or polished surface helps paste release, it may also cause the paste to skip the surface of the stencil rather than roll with the squeegee. Stencil manufacturers address this problem by polishing the aperture walls selectively, but not the stencil surface. While nickel plating improves stencil smoothness and the printing performance, it reduces the aperture opening, which requires artwork adjustment.
Laser Cutting for Stencils
Laser cutting is a subtractive process, where Gerber data is fed to a CNC machine that controls the laser beam. The laser beam starts from inside the boundary of the aperture and traverses to its perimeter, while completely removing the metal to form the apertures, one aperture at a time.
Several parameters define the smoothness of the laser cut. This includes cutting speed, beam spot size, laser power, and beam focus. Typically, the industry uses a beam spot of about 1.25 mils, which cuts very accurate aperture sizes over a wide range of shape and size requirements. However, laser cut apertures also require post-processing treatments just as chemically etched apertures do. Laser cut stencils require electropolishing and nickel plating to smoothen the inside walls of the aperture. As the aperture size reduces during the latter process, the laser cut aperture size must be suitably compensated.
Aspects of Printing with a Stencil
Printing with a stencil involves three distinct processes. The first is the aperture-fill process where the solder paste fills the aperture. The second is the paste transfer process, where the paste accumulated in the aperture transfers to the PCB surface, and the third is the positional location of the deposited paste. The three processes are vital to achieving the desired result—depositing a precise volume of solder paste (also called a brick) to the correct location on the PCB.
Filling the stencil aperture with solder paste requires a metal squeegee blade forcing the solder paste into the aperture. The orientation of the aperture with respect to the squeegee blade affects the fill process. For instance, apertures oriented with their short axis in the direction of the blade stroke fill better compared to those with their long axis oriented to the blade stroke. Additionally, as squeegee speed influences aperture fill, lower squeegee speeds achieve better fills for apertures with their long axes oriented parallel to the stroke of the squeegee.
The edge of the squeegee blade also influences the way the paste fills the aperture of a stencil. The usual practice is to print applying minimum squeegee pressure while maintaining a clean wipe of the solder paste on the stencil surface. Increasing the squeegee pressure may damage both the squeegee blade and the stencil, while also causing paste smearing below the stencil surface.
On the other hand, lower squeegee pressure may not allow release of paste through a small aperture, resulting in insufficient solder on the PCB pad. Additionally, paste left on the side of the squeegee near large apertures will likely be pulled down by gravity, resulting in deposition of excess solder. Therefore, a minimum amount of pressure is necessary, which will achieve a clean wipe of the paste.
The amount of pressure to be applied also depends on the type of solder paste being used. For instance, Teflon/nickel-coated squeegee blades require about 25-40% more pressure when using lead-free solder paste than that required for tin/lead paste.
Performance Issues with Solder Paste and Stencils
Certain performance issues related to solder paste and stencils are:
  • Thickness of the stencil foil and the aperture size determine the potential volume of solder paste deposited on the PCB pads
  • The ability of the solder paste to release from the aperture walls of the stencil
  • Positional accuracy of the solder brick printed on the PCB pad
During the print cycle, as the squeegee blade travels across the stencil, solder paste fills the stencil aperture. During the board/stencil separation cycle, the paste releases to the pads on the board. Ideally, all the paste that filled the aperture during the print process should have released from the aperture walls and transferred to the pad on the board, forming a complete solder brick. However, this transfer amount depends on the aspect ratio and area ratio of the aperture.
For instance, with an area of the pad greater than two-thirds the area of the inside aperture wall, the paste can achieve a release of better than 80%. This means reducing the stencil thickness or increasing the aperture size can give a better paste release with the same area ratio.
The ability of the solder paste to release from the aperture walls of the stencil also depends on the finish of the aperture walls. With electropolished and/or electroplated laser-cut apertures, the paste transfer efficiency improves. However, transfer of solder paste from the stencil to the PCB also depends on the adhesion of the paste to the aperture walls of the stencil and on the adhesion of the paste to the pad on the PCB. For a good transfer the latter should be greater, which means the printability depends on the ratio of the stencil wall area to the open face area, while ignoring minor influences such as the draft angle of the wall and its roughness.
The positional and dimensional accuracy of the solder brick printed on the PCB pad depends on the quality of the transferred CAD data, the technology and methods used to manufacture the stencil, and the temperature of the stencil during its use. Moreover, the positional accuracy also depends on the alignment methods used.
Framed Stencils or Glue-In Stencils
Framed stencils are the strongest form of laser-cut stencils available and are designed for high-volume screen-printing during production runs. These are permanently mounted in a stencil frame with a mesh border tightly stretching the stencil foil taut in the frame. Framed stencils with smooth aperture walls are recommended for Micro BGAs and for components with 16 Mil pitch and below. Framed stencils offer the best positional and dimensional accuracy when used under controlled temperature conditions.

Fig.3: Framed Stencil

Frameless Stencils

For short runs or prototype PCB assembly, frameless stencils offer optimum solder paste volume control. They are designed to work with a stencil tensioning system, which are reusable stencil frames such as universal frames. As the stencils are not permanently glued into a frame, they are significantly cheaper than framed stencils and take up considerably lower storage space.
Conclusion
For achieving good printing results with a stencil, a variety of factors must form a right combination. These include:

  • The right paste material—correct metal content, viscosity, the largest powder size, and lowest possible flux activity
  • The right tools—proper stencil, printer, and squeegee blade
  • The right process—good registration, and clean sweep.

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Taking Care of Warpage and Thermal Profile Issues during Assembly

A warped printed circuit board (PCB) is one that does not sit true on a flat desktop. Although there are several reasons as to why this should happen, there are two specific causes that cause board warping. One of them is layout related, while the other is process related. If the PCB is found to be warped before the start of assembly, the problem has occurred between layout and fabrication. If the PCB was flat before, but was found warped after assembly, then the problem is likely between fabrication and assembly. However, some fabrication problems will not show up until the PCB has passed through the reflow oven.

Fig.1: Warped PCB

Settling on the root cause is generally an iterative process. To discuss the issue with the fabrication or assembly shop, collecting some additional information is necessary. This could be the amount of warpage per inch, size of the board, and its thickness. Along with this information, it may be necessary to consider copper pours, placement of components, and their sizes, provided the board design is in-house.

With the above information, a discussion with the design house, fabrication, or the assembly shop may serve to highlight the issue and cause of the warpage. Once the design issue is ruled out, it may be necessary to determine if it relates to fabrication or assembly, which leads to the next step of determining a solution to the problem.

Design Issues Contributing to Warping

There can be several obscure design issues contributing to PCB warping. To be able to eliminate them before moving over to fabrication or assembly, it is necessary to know the major design contributors:

  • Odd shapes or large cutouts—can cause considerable warping at any stage
  • Thin board—related to number and size of components can lead to warping after assembly
  • Heavy components grouped together—can cause warping during assembly. The thermal mass acts like a heat sink does, leading to uneven expansion and non-even soldering on the PCB
  • Uneven copper pour—although copper and Polyimide/FR-4 are matched for thermal expansion, the matching is not exact. The dissimilarity is magnified for a large copper pour on one side or on a corner, leading to warping either during fabrication or during assembly. This may require the designer change over the solid copper pour to crosshatch for reducing the warpage.

Fig.2: Solid Copper Pour & Crosshatch

Assembly Related Issues Contributing to Warping

Polyimide and FR-4 both absorb moisture from the environment. If a stack of bare boards have been stored for some time before assembly, it is customary to bake them for at least 3 hours at 80-150°C to drive out the moisture. Assembly can start once they are cooled to room temperature. Some board suppliers vacuum seal their products with a desiccant before shipping, and if the complete package is not consumed in one assembly run, it is necessary to reseal the balance along with the desiccant.

Warping in large boards may be related to the orientation as the boards enter the reflow oven. Placing the larger dimension parallel to the conveyor of the oven can prevent uneven heating between the edge of the PCB and its middle. Large boards may require additional support at the center of the PCB, and adding carriers may be necessary. Similar support may also be required for flexible PCBs during reflow.

Fig.3: Breakaway PCB

Fig.4: V-Groove on PCB

Often small PCBs are grouped together to form panels for more efficient assembly. After assembly, operators manually separate them into individual PCBs. Manufacturers follow two methods for easing the process of separation—breakaway and v-scoring. In breakaway, most of the material between the smaller PCBs is cut away, leaving them joined by only small lands. If the panelization is large, these cutouts could lead to warping. V-scoring is an alternative method where instead of removing a part of the PCB, a v-shaped groove forms the separation line. This prevents warping of the panel during assembly, but allows the operators to separate the boards easily.

Fig.5: Reflow Oven

Although 3-zone reflow ovens were adequate for tin/lead soldering, with the advent of RoHS processes and lead-free soldering, reflow ovens operate at higher temperatures. Moreover, to reduce thermal shock to the PCB and components when in reflow, the temperature is gradually built up over several zones, sometimes as many as six to nine—with greater possibilities of error in the settings in one or more of the zones.

Warping may also be caused by oven loading. If the thermal profiling for a specific PCB was done for a certain number of boards in the reflow oven at the same time, the oven temperature may rise when in a subsequent run the number of boards is smaller. As the number of boards in the oven influences the thermal load, the thermal demand may not be adequate to maintain the typical reflow profile for the smaller batch of PCBs.

Clogging of airflow outlets of the reflow oven may also cause a change in the temperature-time profile for a specific PCB, resulting in uneven heating and consequently to warping. Likewise, clogging of orifice plates due to flux accumulation may also limit the forced convection flow, causing the boards to heat up unevenly.

Preventing Warping of PCBs

For PCBs meant for surface mount technology, the IPC-6012 standard defines the maximum camber and twist or warping to 0.75%, while for other types (through-hole technology) this is relaxed to 1.5%. However, most electronic assembly plants dealing with double/multilayer boards prefer to limit warping to between 0.7 and 0.75%. Rigid PCBs with thicknesses of around 1.6 mm and using SMDs and BGAs can only stand warping to the extent of 0.5%, while others using PoP can handle warpages of only 0.3% or less.

Storing PCBs properly before assembly is crucial in preventing warpage. Stacking PCBs on their edge cannot guarantee they will be vertical all the while, and the combined weight of the PCBs will ultimately cause a camber. Therefore, storing them horizontally on a flat surface is essential.

It is usual for PCBs to absorb moisture from the surroundings. Storing them in an area where the temperature and humidity is under control is the ideal solution. Where it is impractical to control the environment within the entire store, use of desiccants to control the moisture within a sealed container may also help in preventing PCBs from acquiring moisture during storage.

As PCB fabricators use heat to form multilayer boards, re-application of heat to warped bare boards should be helpful in straightening them as well. This may require heavy-pressing the boards between heated smooth steel plates for 3 to 6 hours and baking the PCBs 2 to 3 times.

Impact of Quality of PCBs on Assembly and Product Life Cycle

Irrespective of whether the electronic product is a next generation computer system or a simple mobile handset, inside it there is a printed circuit board (PCB). Engineers design PCBs to support and connect electronic components and other hardware inside the product. The PCB usually has conductive pathways holding the electronic components together by soldering, a process for attaching different metals such as tin, silver, gold, and copper together. The process also serves to interconnect all hardware within the product.
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Fig.1: Printed Circuit Board
Printed Circuit Boards (PCBs) must meet three basic requirements to be acceptable for assembly, and to establish products with longevity. According to Clyde Coombs, the author of “The Printed Circuits Handbook,” these three basic requirements are:
  • The physical form of the PCB should match its intended design. Dimensions and placement of interconnection points and the coating on these interconnection points must allow proper component assembly
  • The PCB must provide proper interconnection between components
  • The circuit board must provide adequate insulation between interconnection points that are not to be connected
The above three items must be acceptable and remain of high quality all through the expected life of the product. As there can be several details related to the three requirements, defining the requirement for acceptability and quality for the PCB suppliers is essential to ensure they meet the three requirements. Properly implemented, the quality and acceptance criteria provide all parties a clear picture of the expectations.
Industrial Standards for PCBs
Conforming to industrial standards has the advantage of establishing a common foundation—creating level playing fields—to allow all participants to adhere to as a minimum. Adhering to industrial standards leads to avoiding many chances of failure. Everyone can easily develop knowledge about a common specification rather than interpreting endless numbers of individual company specifications. However, companies may be forced to move beyond the standards for various reasons, as their designs need to advance further than the scope defined by the standards.
Users evaluate PCB quality based on their use in products falling into three categories, called classes. These are:
Class 1: General Electronic Products, such as consumer electronic product
Class 2: Dedicated Service Products, such as those providing uninterrupted service
Class 3: High Reliability Products, such as those providing continuous service
Typically, manufacturers determine the class appropriate for their product. For instance, if a toy manufacturer wants PCBs that meet class 3 requirements, they must be willing to pay for the extra level of reliability.
Internationally, the IPC-6011 standard defines the generic performance specifications for PCBs. According to IPC-6011, the supplier of the PCB is responsible for verification of compatibility with the specifications, master drawings and patterns, and the specific manufacturing facilities and processes. In short, the supplier must ensure the PCB meets the requirements of the procurement documentation.
By default, the IPC-6011 standard applies to all circuit board types. However, this needs to be supplemented by performance specifications containing the requirements of the chosen technology such as:
IPC-6012: for Rigid PCBs
IPC-6013: for Flexible (Flex) PCBs
IPC-6014: for PCMCIA PCBs
IPC-6015: for MCM-L PCBs
IPC-6016: for HDI PCBs
IPC-6017: for Microwave PCBs
Although IPC-6012 is the most common specification used in documentation packages, manufacturers can specify the requirements according to the PCBs their electronic products use.
Acceptability of PCBs
Sometimes, it may be impossible to establish criteria for non-conformance from descriptions alone, such as from IPC-6012 and others. To circumvent this, the standard IPC-A-600 has been developed, which contains illustrations and photographs, and offers three levels of quality for each specific characteristic: Target, Acceptable, and Non-Conforming Conditions. Furthermore, the characteristics are divided into two general groups:
Externally Observable Conditions: these are features or imperfections visible on the exterior surface of the board and it is possible to evaluate them.
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Fig.2: Illustration of Void within a Via
Fig. 2 is the image of a copper plating acceptable for a product of class 2. The acceptance criteria for the plating are:
  • Not more than one void in any hole
  • Not more than 5% of the holes have voids
  • Any void is not greater than 5% of the length of the hole
  • The void is less than 90 of the circumference of the hole
Internally Observable Conditions: these features or imperfections can only be detected, examined, and evaluated after a micro sectioning of the PCB.
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Fig.3: Micro-Section of a Via Hole
Fig. 3 is the image of a plated hole, showing in micro-section the thickness of copper and insulating layers, along with highlighting of material imperfections, if any.
The thickness of copper and insulation layers must match those specified in the design documents. Overall thickness of the board is also an important criterion.
Typically, such micro-sections are performed on coupons with the same characteristics as possessed by the actual board design. This avoids destroying a good board while testing.
General Defects in PCBs and Their Effect on Assembly
Board Warpage: If the board is not perfectly flat, it can cause several problems in the assembly process. This could be a local change in the PCB thickness or an issue of coplanarity of the PCB. It can result in potential opens from tilted components known as the teeter-totter effect, or from dropped solder connection, such as with a BGA joint. A solder joint may also potentially lose reliability from stretching. In general, leadless devices are more susceptible to PCB warpage.
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Fig.4: Ball Drop & Stretched Joints
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Fig.5: Effect of Warpage on Leadless Devices
With a warped board, there may also be an issue with controlling the volume of paste deposition, both for solder paste and adhesive. Warpage usually results in the stencil being unable to sit flat all over on the PCB surface, leaving gaps in between the stencil and board in certain areas. While this may result in uneven printing of solder paste across the board, there may be insufficient adhesive dispensing or adhesive may be skipped altogether in those areas.
While printing, gaps between the stencil and board may fill up with solder paste. This may cause a smear, wet bridge, or excess deposit of solder paste. Sometimes, paste fringing on the stencil after printing may cause unnecessary smears on the next print.
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Fig.6: Unsoldered Leads
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Fig.7: Tombstoned Passive Component
Uneven deposition of solder paste may result in insufficient volume being present, showing up as solder covering the pad, but with metallization showing through. As there is insufficient paste to touch the component leads, it results in unsoldered leads after reflow. For small packages, which usually decrease the total tolerance of PCB and assembly process, this may also lead to paste misalignment, resulting in cocking or tombstoning of passive chip components, impacting process yields.
During waves soldering, if via holes are unfilled and the board has warpages, the assembly can potentially lift off the wave, resulting in areas remaining unsoldered.
Solder Mask Issues: Improperly applied solder mask may cause reliability issues during assembly. One of the major issues is the missing solder mask dam between two neighboring solder pads, leading to a potential solder short or bridging during a wave soldering operation.
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Fig.8: Missing Solder Mask Dam
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Fig.9: Shifted Solder Mask
Normally, the solder mask is required to cover a pad all around. If there is a shift or misregistration, the solder mask may not cover the land fully, and form a pocket next to the land, exposing the neighboring pad or track. This area can subsequently fill with solder paste and create a whisker or bridge shorting the pads with the neighboring pad or track.
Another reliability issue arising from improperly applied solder mask is a smear on the pad itself causing solderability issues. This prevents solder from wetting the smeared land properly during reflow or wave soldering, leaving the pad non-solderable.
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Fig.10: Solder Mask Smear on Pad
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Fig.11: Solder Ball
 
If the solder mask remains under-cured, it can lead to product reliability issues, as areas of under-cured solder mask can trap processing residues and contribute to electrical leakages or to electromechanical migration failures. Improperly cured solder resist may also allow solder to accumulate in the form of balls during wave soldering, leading to potential electrical shorts.
Issues with HAL Boards: Non-coplanar or uneven solder surfaces are a major issue for HAL finish boards. A very thin coating of HAL may lead to migration after the first reflow operation, exposing copper and leading to poor solderability in subsequent reflows.
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Fig.12: Uneven HAL Solder
 
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Fig.13: Thin HAL Coating
 
As stencil openings normally do not match the pad perfectly, some parts of the stencil may rest on the excess solder deposit, leaving the other area of the stencil lifted away from the board. This allows solder paste to squeeze into the gap between the stencil and the board, creating issues similar to those on boards with a warpage.
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Fig.14: Solder Bridge
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Fig.15: Grainy Solder Joint
Non-coplanarity of HAL boards may also lead to bumps of solder being left on pads causing the stencil to lift up and solder paste filling the underneath gap. During reflow, the excess solder may cause adjacent lands to bridge or whiskers to form between adjacent fine-pitch components. The flux in the solder paste may not be adequate for the total amount of solder from the paste and that left on the pad by the HAL process, and this may result in a grainy or disturbed joint.
Conclusion
The PCB being an electromechanical item in a product has many opportunities for failure. PCB quality is a huge subject with numerous possibilities for imperfections affecting assembly and the product life cycle. Starting from warping of the PCB surface to via failure to under-etching of traces, each aspect of PCB quality can bring the product to an abrupt halt much before its intended life cycle is over.
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PCB POWER launched online High Quality Power Stencils

PCB POWER has now decided to start shipping high quality laser stencils for its customers. Well-known for being ultimate in class service and reliability, PCB POWER will now be providing its customers stencils on demand.  The customers would be free to choose the type of stencils (framed or frameless) they want and place an order for the same online. They can also avail the benefit of a separate price calculator having user friendly concept created for this purpose.
Stencils are used in assembly of high reliability surface mount component assembly. For ultimate accuracy, we manufacture our stencils using fine quality steel and ultra accurate cutting technology.
Customers can now conveniently order PCBs along with Stencil. For further details, please login on your account http://login.pcbpower.com/V2/login.aspx
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RF Design and High Frequency Board Manufacturing

The performance of a product operating at high frequencies depends largely on the electrical characteristics of the Printed Circuit Board (PCB) used for mounting and connecting its circuit components. The magnitude of the impact of the PCB design increases exponentially with increase of the operational frequency. Therefore, designers need to include electrical models of PCB structures when simulating RF circuits. For achieving optimum solutions, the product/PCB designer and the manufacturing engineer must appreciate the requirements of RF design.
Designing for High Frequencies
Designing a board to work at high frequencies requires the designer to be critical of the following areas:
  • Material used for the PCB
  • Placement of traces
  • Placement of planes
  • Component interconnections

Materials Used for RF PCBs

RF PCBs can use a variety of different materials. Although common board materials used for high frequency circuits are FR-4 and derivatives of FR-4, many other base substrates are also used as they offer better electrical performance. These include specialized low-loss RF material such as pure PTFE, ceramic filled PTFE, Hydrocarbon Ceramic, and High-Temperature Thermoplastic/Ceramic.
Although FR-4 has its limitations when used for high-frequency work, the RF designer must understand these limitations and make cost/performance tradeoffs for the design. Typical limitations of FR-4 are:
  • Stability of dielectric constant—Varying from lot to lot and over frequency
  • Loss factor—Depending on surface contamination and the hygroscopic nature of the material
  • Ability to withstand processing temperatures—Lead-free processing temperatures are higher than regular soldering temperatures
  • Thermal conductivity—Even low-power RF circuits can produce a lot of heat

Therefore, selecting a suitable material for making a PCB operating at high frequencies depends on the above factors and the product cost. The choice could range from the low-cost FR-4 material, with its higher loss and not tightly controlled dielectric constant, to FR-4 derivatives with better specifications, or to other specialized low-loss RF material with their well-specified dielectric constant.

Fabrication Issues with Special Materials
 
All laminates mentioned above involve individual fabrication issues. For achieving the proper quality and reliability, the manufacturer must follow these individual fabrication notes for each substrate material for storing, handling, preparing the inner layer, surface preparation for photoresist application, bonding, drilling, deburring, and plating.
Manufacturers require setting up special processes for fabricating PCBs with low-loss RF materials to work at high frequencies. For instance, plated-through hole preparation is very critical for PTFE substrates—it needs an etch-back process requiring Plasma etch setup to prepare the PTFE hole surface and make it capable of accepting electroless copper plating. Therefore, apart from proper selection of material, following the proper fabrication methods is equally important for achieving a good quality PCB working reliably at high frequencies.
Placement of Traces
For matching the impedance, designers effectively manage the spacing of traces, ground planes, and the dielectric material to form a controlled impedance transmission line. They do this in several ways—in the form of a microstrip, stripline, co-planar waveguides, and differential pairs. The width of the trace, the dielectric thickness, dielectric constant of the used dielectric material and copper thickness determine the impedance. As high frequency signals are very sensitive to noise, ringing, and reflections, they must be designed with great care towards impedance. Mostly preferred impedance is 50 ohms for single ended and 100 ohms for differential, with control limits of ±10%.
                                                               Fig.1: Microstrip
                                                         Fig.2: Centered Stripline
                                                               Fig.3: Off-Center
Microstrip: This is a circuit trace carrying the RF signals routed on an outside layer of the PCB with a reference plane below it. The reference plane may be power or ground plane.
Stripline: This is a circuit trace carrying the RF signals routed on an inside layer of the PCB with two low-voltage reference planes above and below it. The reference planes may be power and or ground plane. The stripline can be equidistant from the two reference planes, in which case it is called the centered stripline, or it can be an off-center stripline, where it is closer to one of the reference planes.
                                                               Fig.4: Coplanar
                                              Fig.5: Coplanar Waveguide with Ground
Co-planar Waveguide: This is a circuit trace carrying the RF signals embedded within a ground reference plane on the same layer of the PCB. Co-planar waveguides (CPW) offer lower loss tangent than microstrips do, but have a higher skin effect loss, as fields concentrate on the edges of the trace and ground. Another form of co-planar waveguide is the co-planar waveguide with ground (CPWG), where a ground plane is placed just below the waveguide layer.
                                                    Fig.6: Coplanar Differential Pair
                                                  Fig.7: Coplanar Differential Pair with
 
Co-planar Differential Pairs: These are two traces carrying the RF signals embedded within a ground reference plane on the same layer of the PCB. This arrangement is also called the CP Differential Pair or Edge-Coupled CPW. This gives an extra degree of signal-to-noise isolation over the standard CPW. An added ground plane just below the layer offers even better field containment over the coupled CPW, and is called the Edge Coupled CPWG.
Placement of Planes
Most RF products use multilayer PCBs. These comprise a number of laminates of the substrate material separately etched, drilled, and bonded. The chief advantage of this is to allow the use of more than two conductor layers, thereby reducing the required board space, but at increased cost.
Setting up the laminates is a major part of the design for a multilayer RF board. The stack defines the number of layers the board will ultimately possess. At this stage, it is important to define the layers carrying specific high-speed tracks, and the placement of the ground and power layers with respect to those layers. Enclosing tracks carrying high frequency signals within the ground and power layers serves to define two significant factors related to high speed multilayer design—minimizing cross-talk, and maintaining a check on the impedance on the board. However, the cost of the board increases proportional to the number of layers it has, and therefore, the number of layers is usually a compromise of the board’s functionality and its cost.
RF products typically use a four or six layer FR-4 multilayer construction. Drilled and plated through holes or vias link tracks on one layer to tracks on other layers or all layers. Complex structures use blind or buried vias, with blind vias connecting the outermost layers to one or more inner ones, while buried vias connect only the inner layers and do not appear on the outermost layers. The third type of via is the through via, going through all the layers of the board. To create the connections, it is necessary to drill and then plate-through all vias. Via structures have a major effect on the fabrication processes of the PCB and contribute to the cost of the finished board.
Component Interconnections
Parasitic elements of a PCB refer to its physical attributes that affect the performance of the circuit. For instance, at high frequencies, a long thin track will usually be inductive, while a large pad over a ground plane will behave like a capacitor. In addition, when modeling in real circuits for, say a series capacitor, the designer must also include the impedance of the connections between the ground plane and circuit components.
A plated through via hole also adds significant inductance. RF designers can use good circuit simulation packages that include models to allow their addition. For instance, the typical inductance of a 0.2 mm diameter, 1.6 mm long hole can be as much as 0.75 nH. Although this may seem to be small, it can exert significant influence at high frequencies.
Components mounted on the PCB also contribute with their non-ideal characteristics. The use of Surface Mount Device (SMD) components helps to reduce the effect largely because of their reduced lead lengths and small construction, but the effect is still prominent at higher frequencies.
Designers use different ground plane strategies for their RF PCB design, and there is no unique solution as the best strategy. While most designers advocate breaking up the ground plane over the analog, digital, radio, and audio parts of the circuit, providing an individual ground plane of low impedance for all parts of the circuit is usually a good point to start.
Designers need to consider the flow of currents carefully throughout the product to minimize interferences between the audio and radio circuits. This assumes even greater significance if the design uses Digital Signal Processing (DSP) and microprocessors.
RF PCB Layout Strategies and Techniques
  • Separate all RF, low-level analog,  and digital sections.
  • Divide the RF section into circuit groups (amps, LO, VCO, etc.).
  • Place all the high-frequency components early in the layout, as this helps to minimize the length of the RF routes (in RF PCBs, functional orientation is more important compared to DFM).
  • Place the components carrying the highest frequency next to the connectors.
  • Never place unrelated inputs and outputs next to each other. For instance, multi-stage windings should never be placed adjacent.
  • When long input or output to RF amplifiers is unavoidable, choose to make the output longer.
  • As the trace impedance is a critical factor when trying to control reflections, always match the impedance between the driver and the load, except where the trace is shorter than 1/20th of the wavelength.
  • When using pull-up inductors or resistors at the outputs of open-collector devices, always place the pull-up component next to the output pin it is pulling up.
  • In addition to decoupling the main power pins of the IC, decouple the pull-up also.
  • Inductors usually have large magnetic fields around them-
    • Never placed them close together, when in parallel (unless the intention is to couple their magnetic fields)
    • Separate all inductors by 1x times the body height (minimum) OR
    • Place inductors perpendicular to one another
  • Confine “ALL” routes to the section or stage to which they are assigned –
    • Digital traces in the digital section
    • Low-level analog traces in the low-level analog section
    • RF traces in the RF section
    • Routing traces into adjoining sections is not recommended
  • Route all short RF traces on the component side of the PCB, rout them to eliminate vias
  • Place a ground layer below the RF traces.
  • Minimize the vias in the RF path, as this reduces the breaks in the ground plane(s) and –
    • Minimizes inductance
    • Helps contain stray magnetic and electric fields.
  • Long controls lines are acceptable, but take care to route them away from RF inputs.
  • Keep RF lines away from one another by a minimum distance to avoid unintended coupling & crosstalk.
  • Minimum spacing is a function of the acceptable level of coupling, and is good for crosstalk, directional couplers, crosstalk, differential lines coupled in even or odd modes.
Summary
Finally, the design of a PCB and its fabrication for high frequency use is a complex process requiring intimate communication between the designer and the fabricator, with each understanding the issues related to high-speed design.
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PCBPOWER (Subsidiary of Circuit Systems India Ltd) has come a long way ever since it began to manufacture PCBs in 1996 to become one of India’s leading PCB manufacturers today. The company’s high quality and cost effective printed circuit boards with its unmatched consistency and customer-centricity has earned it respect and appreciation on a global level. PCBPOWER has a long-established reputation in providing its customers prototype and small volume through an efficient usage of the state of the art production facility.

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With the rapid technological and economical changes, only a flexible organisation can withstand the turbulent circumstances. Flexibility has been our forte. We believe in providing solutions that meet the requirement of our customers and are revolutionary at the same time. Among our ground breaking solutions are a wide range of New Advancements in High Frequency-RF PCBs in Metal Clad PCB’s, RT Duroid and Higher Layer counts with lean manufacturing and SPC. We are a UL certified facility.

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