Trends in Surface Mount Technology and Its Relevance with PCB Surface Finish

It is necessary to mass-produce electronic circuit boards in a highly mechanical manner for ensuring the lowest cost of manufacturing. Traditional through-hole electronic components with leads did not lend themselves to this approach. Therefore, since the 1980s, virtually all electronics hardware is being mass-produced using surface mount technology (SMT). Compared to the through-hole technology (THT) used earlier, the surface mount devices (SMD) associated with SMT offer several advantages in terms of manufacturability and performance.

                                               Trends in Surface Mount Packaging
Almost all electronic components are available in forms suitable for surface mounting. SMDs do not have long leads that necessitates passing through the printed circuit board (PCB). Rather, they have very short leads that can be soldered directly to the copper pads on the PCB. Manufacturers use different types of SMD packaging, with the evolution tending towards increasing package and pin densities.

                                           Fig. 1: Trends in Surface Mount Packaging
The popular dual in line (DIP) packaging for ICs with two rows of pins for soldering has now diverged into the PGA, QFP, and TSOP type SMD packages. Compared to DIP, these packages have improved on the packaging density enormously. However, modern electronic equipment design demands even higher densities. As a consequence, we now see extremely dense SMD packaging in the form of BGAs, LQFPs, and TCPs. Now, SMD packages are converging towards chip scale packaging (CSP) types, offering better heat dissipation, higher package densities, and increased flexibility.
This trend towards miniaturization is visible for other passive components as well. All types of resistors, capacitors, and inductors are now available in small SMD packages. For instance, although the 0603 and 0402 packages are most commonly used, smaller sizes of 0201 are also available.
Trends in Soldering Techniques for SMT
Most countries have realized the hazards of using the element Lead in electronic equipment, and as a result, the use of lead and tin combination for production and use of solder has almost stopped. Instead, the industry now uses various forms of lead-free solder, although these have more stringent process requirements.
With the advent of new types of SMD packages, the trend in soldering techniques is also evolving. From the commonly used wave soldering for through-hole devices, the trend is towards use of non-contact soldering using infrared and hot gas reflow methods for SMDs.
Trends in Machinery for SMT
Mass production and high mechanization has replaced manual insertion of through-hole components with sophisticated pick-and-place machines for SMD components. These take the form of precision nozzles, intelligent feeder systems, multi-functional mounters, and 3-D molded interconnect devices.
Apart from advances in automated machinery used for SMT, the introduction of special SMD packages such as BGAs has necessitated use of specialized equipment for inspection of PCBs after assembly. Since it is visually impossible to inspect the underside of a BGA chip after it has been soldered, it is necessary to use X-rays to inspect the soldering. With high volumes of production and miniaturization, it is nearly impossible to inspect PCBs manually after assembly. Therefore, the current trend is towards in-circuit testers (ICT) and computerized automated test equipment using high-resolution digital cameras and special algorithms.
Relevance of Surface Finish of PCBs with SMT
The solderable surfaces of a PCB need protection from oxidation while the PCB moves from manufacturing to assembly. Oxidization of the copper surface prevents formation of a good solder joint. Quality of the surface finish affects first pass yield (FPY) and the final product reliability. Primary reasons for this involve non-uniform surface finish and poor solderability. Although there are other known factors for poor FPY, but surface finish issues are the main.
Typical surface finishes manufacturers use are:
  • Hot Air Solder Leveling (HASL)
  • Organic Solderability Preservatives (OSP)
  • Immersion Silver (ImAg)
  • Immersion Tin (ImSn)
  • Electroless Nickel/Immersion Gold (ENIG)
  • Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
Hot Air Solder Leveling (HASL) is the most common PCB finish, for both lead and lead-free compositions of solder. The process involves application of molten solder to the exposed pads in a vertical or horizontal panel orientation, with excess solder being blown away with a forced hot air knife. The typical thickness of HASL solder on the copper pad ranges from 0.3-1.5 mil, melting at 183°C for lead solder and at 228°C for lead-free solder, with a typical 12-month shelf life.

                                                      Fig. 2: Hot Air Solder Leveling
However, for HDI applications, the HASL process presents a highly variable topography, or inconsistent surface planarity because of the formation of solder beads/balls not conducive to SMT, especially for QFP and BGA packages. In addition, depending on the alloy used for lead-free solder, the HASL process may be aggressive on copper, reducing the shelf life. While the thermal shock may cause warping of the PCB, there can be PTH diameter issues and bridging of fine pitch traces with solder mask residue preventing HASL from flowing. In addition, contamination on the surface of the copper or resin residue on the laminate may cause poor bonding.
Organic Solderability Preservatives (OSP) is a low-cost transparent coating of organic material, which preserves the copper surface from oxidation until assembly. The process involves application in a dip tank with the PCB in a vertical position, or the use of a conveyorized chemical process, which leaves a very thin coating of the material, typically 100-4000 Angstroms thick. Although OSP is a flat, reliable planar surface, well suited to BGA and QFP packages, the shelf life is rather low, being typically 6 months or lower.

                                              Fig. 3: Organic Solderability Preservatives
OSP is difficult to inspect, and does not stand multiple reflows very well. This raises questions of reliability of exposed copper pads after assembly. As OSP is not conductive, ICT test pads need to be soldered.
Immersion Silver (ImAg) is a metallic solderability preservative, and the process deposits 8-15 micro inches of nearly pure silver on the copper surface. Although it provides a flat, planar surface, excellent solderability, and about 6-12 months of shelf life, immersion silver is sensitive to handling, packaging, electrical tests, and suffers from creep corrosion from salt and sulfur in the environment.

                                                            Fig. 4: Immersion Silver
Immersion Tin (ImSn) forms an intermetallic joint with copper to provide a uniform, dense coating with excellent hole-wall lubricity. As it is possible to engineer immersion tin to be non-porous and with very fine grain, it is the top choice for backplane panel assemblies requiring press-fit pin insertions.
However, immersion tin has a shelf life of 6 months, and is sensitive to handling. In addition, processing of immersion tin requires using Thiourea, a carcinogen with environmental issues.

                                                               Fig. 5: Immersion Tin
Electroless Nickel Immersion Gold (ENIG) is a complicated chemical process, involving nickel plating over the copper pad and subsequent gold plating over the nickel. The gold layer prevents the nickel from oxidizing during storage, while also providing low contact resistance, good wetting for solder, and excellent shelf life of typically 12 months. The flat planar surface is well suited for fine pitch devices such as BGA and QFP. Being conductive, ENIG offers good ICT contacts.

                                              Fig. 6: Electroless Nickel Immersion Gold
However, ENIG is an expensive process, with non-wetting issues if the process has not been executed properly. Slow intermetallic growth can result in poor joint reliability and strength.
Electroless Nickel Electroless Palladium immersion Gold (ENEPIG) is another complicated chemical process, involving depositing electroless nickel on the copper surface, followed by a coating of electroless palladium layer, topped with a layer of immersion gold. The triple layer helps to form a superior solder joint with lead-free solder. As the process allows a thinner layer of gold, the process is less expensive when compared to ENIG, although the extra process step offsets this. The flat planar surface suits fine pitch devices such as BGA and QFPs. As the shelf life is typically 12 months, ENEPIG is the fastest growing surface finish.

                               Fig. 7: Electroless Nickel Electroless Palladium Immersion Gold
PCB manufacturers prefer ENIG and ENEPIG to others because of the relative advantages the two techniques offer, although between the two, their advantages vary. ENIG is suitable for SMT, especially for BGA and other fine pitch components. The technology works well for lead-free soldering, and is highly reliable, which is why the flex PCB market prefers ENIG.
On the other hand, ENEPIG has a much wider acceptance and is suitable for multiple types of packages including THT, SMT, wire bonding, press fit, and more. Apart from being suitable for fine-pitch SMD components such as BGA and QFPs, ENEPIG is applicable to PCBs with different manufacturing technologies, requiring higher densities and reliability.

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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.
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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FRAMELESS STENCILS – Production & Benefits

Frameless Stencils which are loose foil not with rigis Frames have the advantage of being extremely economical and save tremendous amount of space as well. For Limited Production runs and where budgets are a factor, these are very useful.

1. Patterned border
2. Basic outline border.


Frameless Stencils are laser sliced weld glue stencils intended to work with stencil tensioning frameworks otherwise called Reusable Stencil Frames. Frameless Stencils are noteworthy more affordable than Framed Stencils and lessen storage room necessities.

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Online High Quality Power Stencils

PCB POWER has now chosen to begin transporting great power stencils for its clients. Surely understood for being extreme in class administration and dependability, PCB POWER will now be giving its clients stencils on request. The clients would be allowed to pick the kind of stencils they need and put in a request for the same on the web. They can likewise profit the advantage of a different value number cruncher having easy to understand idea made for this reason.

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Stencils are utilized as a part of gathering of high unwavering quality surface mount segment get together. For extreme exactness, we produce our stencils utilizing fine quality steel and ultra precise cutting innovation.


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Online PCB Specialist – pcbpower

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.

Our customers are invaluable to us; their time, money and efforts matter a great deal. We, therefore, strive single-mindedly to minimize the cost, time and efforts of our valued customers, providing them complete solutions concerning PCB fabrication, PCB Layout-Design, Stencil Fabrication through our online ordering & tracking system.

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