Relationship between Solder Wicking and Surface Finish

During reflow, the solder paste may tend to wet the component termination rather than both the printed circuit board (PCB) pad and the lead, as it should do normally. This phenomenon is called solder wicking, and the main reason for this is the slow wetting of the pad or a much faster wetting of the termination.

Fig.1: Mechanism of Solder Wicking

Faster Wetting of the Termination

The reflow profile or the soldering process is the main cause of faster wetting of the component lead. For instance, vapor phase can allow the leads to reach soldering temperatures much before the surface of the PCB can. Another contributor to wicking is the old tin/lead coated termination in a lead-free process. As the melting point of tin/lead coating is lower, the surface coating wets very quickly. The result literally sucks solder up the lead before it can even start to wet the pad on the PCB. Although in most cases the joint is satisfactory, larger component leads may carry the solder up, creating incomplete joints.

Slow Wetting on the Pad

Most lead-free alloys have reduced wetting characteristics, and the alternative surface finishes exaggerate these characteristics. Manufacturers improve the solderability performance of the PCB surface by coating them with other metals. A coating of solder offers the best solderability, followed by gold, silver, tin, and copper OSP. Of course, this is based on the specification, quality, and consistency of the finish provided. Several factors may contribute to the degree of surface coverage. Engineers need to optimize their reflow process for obtaining the best performance from the PCB.

Surface Finish Affects Wetting

In general, the amount of lead-free solder wicking depends on the surface finish of the pad and the finish of the component lead. This may result in unsoldered areas and may result in solder displacement to areas close to the joint with the potential to create shorts. De-wetting may also cause solderability problems, where solder joints can be made and then destroyed by inappropriate soldering conditions.

Usually, manufacturers preserve the solderable surface of the PCB using a surface finish or coating as the PCB moves from manufacturing to assembly. The most common surface finishes offering the best solderability are, in order of preference, Hot Air Solder Leveling or HASL, Electroless Nickel/Immersion Gold or ENIG, Immersion Silver or ImAg, Immersion Tin or ImSn, and Organic Solderability Preservatives or OSP.

HASL—Hot Air Solder Leveling

Fig.2: Hot Air Solder Levelling

As a solder coating, HASL is available for both lead and lead-free PCBs. The application process involves pre-cleaning the exposed copper surface, preheating it, and flux coating it, before giving it a coating of solder. Excess solder is removed and the surface leveled by a hot-air knife. Typical thickness of the HASL layer is usually about 100-500 micro-inches.

Although HASL offers long shelf life, reliability, and very good solderability due to its short wetting time, it can lead to several undesired side effects. HASL may cause uneven surfaces or planarity issues and PTH diameter issues and bridging for fine pitch prints. The PCB may face a thermal shock due to the HASL process, and this may cause PCB warpage.

ENIG—Electroless Nickel/Immersion Gold

Fig.3: Electroless Nickel/Immersion Gold

This is a plating of nickel over the exposed copper pad and a plating of gold over the nickel surface. The application process involves cleaning the exposed copper surface with micro-etching and depositing the electroless nickel over it. A plating of immersion gold then covers the nickel surface. The typical thickness of the nickel layer is 50-150 micro-inches, while the gold thickness is about 3-10 micro-inches.

As the gold surface does not oxidize, it offers long shelf life and good wettability and solderability. However, there are side effects also, such as nickel corrosion leading to black pads, and thick gold surface causing embrittlement. Additionally, slow intermetallic growth of nickel may lead to poor joint strength and reliability. ENIG is an expensive and complicated process, which, if not done right may aggravate non-wetting issues.

ImAg—Immersion Silver

Fig.4: Immersion Silver

Immersion silver of 3-12 micro-inch thickness covers the exposed copper pad on the PCB, which has been cleaned and micro-etched prior to the application of the immersion silver.

Although immersion silver finish is low cost and produces a planar surface of high conductivity, it is subject to creep corrosion, planar micro voids, migration of silver, and tarnishes easily.

ImSn—Immersion Tin

Fig.5: Immersion Tin

Manufacturers often protect the exposed copper pads on the PCB with a deposit of a thin layer of immersion tin. This low cost protection is low cost and produces a uniform surface of good solderability. However, many solder mask materials and plating chemicals attack immersion tin. Tin forms whiskers, which tend to create shorts, and it also forms solid state IMC.

OSP—Organic Solderability Preservatives

Fig.6: Organic Solderability Preservatives

OSP is a transparent organic material coating, which manufacturers use to prevent the exposed copper pads from tarnishing. The applied thickness is usually 0.3-0.5 microns, and it forms an intermetallic bond with the copper surface.

The OSP application process is low cost, easy, and short. Although it produces a flat planar surface of good reliability, it produces the least solderability among all the above surface finishes. It has a short shelf life, and does not withstand multiple reflows.

Solving Solder Wicking in Reflow Soldering

Fig.7: Actual Solder Wicking

As soldering is a complex science, various soldering processes have some common underlying issues and a few unique ones that need to be overcome for achieving a good solder joint. Solder wicking is one such serious problem where the issue is of solder climbing up on to the lead of a component. The root cause and mechanism of solder wicking are:

  • The lead reaching the liquidus temperature before the pad does, tightens up the molten solder completely
  • The hotter surface attracts the solder alloy, provided there is sufficient wettability
  • Solder on the leads melts faster than on the solder joint—the enhanced liquefaction of solder at the lead pulls the melting solder paste away from the pads
  • Wicking-up is aided by all non-wetting problems such as plating too thin, contamination/oxidation, or surface treatment

Experiments with 8-zone reflow ovens have demonstrated good solutions to the problem of solder wicking. Most effective settings for the oven were five pre-heat zones, and three peak zones, along with forced convection airflows. Such reflow ovens were capable of suitably processing very complex boards with optimum process stability and offered unmatched reproducibility.

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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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