Pallets are a solution to odd-form packages.

Time was, all assemblies were designed with the same basic construction. The primary side was where plated through-hole components and certain surface mount parts were installed, while the secondary side had exposed pins for PTH components and might have some SMT components. Most assemblies were processed the same way. Glue dots were applied to the secondary side, SMT components installed, and the glue cured (if SMT components were present). Then, solder paste was applied to the primary side, SMT components installed, and the solder reflowed. Through-hole installation and wave soldering followed. Contending with PTH components meant controlling solder shorts and ensuring design engineering understood how to optimize the design for wave soldering.

Today’s designs are highly integrated, with the vast majority of parts taking SMT form. However, some parts – such as beepers, switches and user-access connectors – don’t lend themselves to an SMT package. Multi-image panels are utilized for many designs to maximize the efficiency of the PCB construction and assembly. This can make it impossible to control coplanarity of the entire panel due to its overall width. All these factors are driving adoption of selective soldering processes.

Masking pallets can generally only accommodate a single assembly per pallet design, so care must be taken to ensure a relatively stable design before investing heavily. Depending on the size of the wave solder machine, typically no more than 10 pallets are required for a fully developed application. Pallets can force designs to use larger keep-outs for non-wave parts, as the solder is indiscriminately applied to the entire exposed area. The pallet must be completely seated to the board to prevent solder leakage into areas that should be shielded from the wave. This requires a sufficient clear area around the exposed portion of the assembly to permit sealing with the pallet.

Reasons for requiring a selective solder pallet are varied. In some instances, a pallet may be required due to SMT components on the secondary side that cannot be wave-soldered or that are installed using a standard double-side reflow process that would be pushed off by the solder wave (due to the lack of glue). This is especially true for designs that have a high percentage of SMT components, with a very small amount of parts that are available only as through-hole. In other designs, a multi-image panel may bow when supported only by the wave solder finger conveyor due to the width of the panel and the mass of the parts. If the individual designs don’t permit the panel to be broken into smaller strips (either due to irregular edge designs or lack of edge clearance for finger conveyors), a pallet can be used for support and/or masking. Finally, masking pallets can permit through-hole components to be installed on both sides of an assembly by using one pallet for each side, and wave soldering the assembly twice.

Processing a masking pallet assembly over a solder wave requires some adjustments to standard conditions. Most important, the preheat temperatures require adjustment, as the pallet adds significant thermal mass to the assembly. Care must be taken to ensure that flux is activated and the assembly does not suffer from thermal shock, as is true for a standard wave solder application. Wave height settings require modification as well. In the case of a standard assembly, the maximum wave height is the height that does not flood the topside of the board. With a pallet, the wave is required to flow into a pocket, so it must be set higher. The pallet’s extra thickness permits the wave height to be higher without flooding the assembly.

Many options exist for manufacturers that find themselves forced to create designs that violate the old ways of processing assemblies. Selective solder pallets permit a manufacturer to capitalize on existing equipment and process knowledge. 

ACI Technologies Inc. (aciusa.org) is a scientific research corporation dedicated to the advancement of electronics manufacturing processes and materials for the Department of Defense and industry. This column appears monthly.

With proper technique, immersion silver corrosion can be remediated.

Immersion silver board finishes, used for protecting copper surfaces prior to assembly, provide a flat, plated surface, which permits superior solderability and has the potential for a long shelf life. However, the exposure of the silver finish (from all vendors) to a sulfur-rich environment with moderate humidity can cause a very distinct chemical reaction, identified as “creep corrosion.” As defined by Dr. Randy Schuller (“Creep Corrosion on Lead-Free Printed Circuit Boards in High Sulfur Environments,” SMTAI, October 2007):

This corrosion of the silver and copper surface is primarily copper sulfide (Cu2S) with a small amount of silver sulfide (Ag2S). Studies have shown that high amounts of Cu2S typically indicate the presence of active sulfur compounds such as elemental sulfur, hydrogen sulfide (H2S), or organic sulfur compounds. Creep appears to begin by growth of dendrites, as shown on an HDD in an early stage of corrosion. However, this is not electro-chemical migration (voltage potential driven) dendritic growth, since creep takes place equally in all directions and does not require the board to be powered. Rather, it appears that Cu2S is being formed in a layer of moisture on the surface and precipitates out of solution as it forms (since Cu2S is insoluble in water).

Therefore, in an uncontrolled product application environment with moisture and high sulfur airborne contamination present, the exposed silver surfaces are susceptible to a creep corrosion event. The Cu2S crystals that form are not just a corrosion issue that can cause an open circuit, but are conductive crystals capable of carrying enough current to short-circuit the hardware (Figure 1). The corrosion areas only occur on the test pads, top of vias and non-soldered plated through-holes. The areas where the immersion silver surfaces are soldered, with no exposed silver, and an intermetallic bond has formed with the copper surface, will not experience creep corrosion; however, if the areas of the pad did not wet completely, creep corrosion can occur.


Can assemblies that have developed creep corrosion and electrical shorting be cleaned? After a period of research and development, we were able to configure a cleaning process capable of removing the Cu2S and corroded silver. The cleaning process did not damage the soldermask, marking inks, solder joints or components, but was effective in removing the creep corrosion and recovering the hardware (Figure 2). Our client, after cleaning, applied conformal coating to the assemblies, put some through environmental testing and returned the remaining assemblies to the field. Test data and product performance in the three years since have been positive. Using conformal coating, the client was able to create a protective barrier that minimized moisture collecting on the assembly surface.

Prior to cleaning the assemblies, we first assessed if residues were present through C3 and ion chromatography. Data indicated the boards were high in sulfate residues, well above recommended cleanliness limits. Data collected after cleaning showed the sulfate levels dropped to undetectable levels, along with a decrease in all other anions (Table 1). The images are of the vias before cleaning and after cleaning.

Creep corrosion failures can be cleaned, hardware recovered, and put back into service. It is important to clean and remove contaminants prior to the conformal coating process to ensure long-lasting product performance.

Terry Munson is president and founder of Foresite Inc. (residues.com); terrym@residues.com.

Setup isn’t easy, but the tester is only as valuable as its implementation is correct.

Any test or inspection tool is only as good as its implementation on the production floor. Without a carefully planned implementation and continuous monitoring of the implementation, the customer will not realize 1) increased ICT and/or functional test yields and 2) an overall cost savings associated with defects being detected and repaired early on in the production process.

Some – emphasis on some – of the key components to proper system implementation include the following:

Programmer training. The biggest factor in ensuring optimum performance of the test/inspection tool is ensuring a good program is generated. This starts with ensuring that your programmer gets the required training and hands-on experience with the equipment. Training may seem like a non-value added task, but doing this ensures you get the highest value out of your equipment. If an experienced user, there are training programs in which the programmer skill level is audited to identify areas of weakness in program development. Custom classes are then created that focus on the developer’s weaknesses so as to improve program quality moving forward.

Program auditing. To ensure programs are of high quality, ideally they will be audited. Audits are not necessary on all test programs, but can be done periodically to ensure the highest quality, while providing ongoing training for the programmer. Third-party providers or internal audits can be used.
Repair operators training. Repair operators are a vital part of the implementation. A company can use a highly skilled 5DX programmer; however, unless the repair operator is trained to properly disposition those solder joints the tester has indicated as either true or false calls, the tester will not drive improvement in assembly quality. It is important operators are trained in image interpretation, and this training is continued as new package types (such as QFNs or stacked BGAs) are introduced.

Monitor false call rate. Post-repair results from the repair station can be used to determine the source of high false calls. Too many false calls results in repair operators ignoring or missing real defects. For this reason, good test inspection tool implementation will include review and correction of false calls. This starts with a review of false call rates of the different boards, prioritization of the boards, and then analysis of the Pareto of false calls. Monitoring and improving the board program by false call analysis will ensure the most value from the test machine, and will minimize the chance that real defects get to customers.

Feedback loops. This is one of the most important aspects of the tester implementation, yet it often gets ignored. Test results from equipment such as the 5DX can be used to provide feedback to engineers on design and process improvements. For example, during the first prototypes run, a review of solder joint images can identify process problems, and can also dramatically speed the launch of a new product. In addition, feedback from downstream test/inspection equipment, such as ICT/FT, can be used to prevent future solder-related escapes from making it to the end-user or consumer by alerting the 5DX engineer of program weaknesses (e.g., solder escapes found at ICT/FCT that were not detected by the 5DX). Thus, defect integration and analysis should be used to improve the overall manufacturing process, as well as the test programs. It is critical there is continuous communication between the various manufacturing groups.

System maintenance. All test/inspection tools require ongoing maintenance to ensure proper performance. For the 5DX, this translates into performing weekly photometric, bimonthly confirmation & adjustments (C&A), and semiannual preventative maintenance. In addition, the 5DX requires a tight maintenance schedule. If this is not done, the machine’s thickness tables will likely drift, creating a need to update and revise all programs developed during this “drift” period. For test/inspection tools, it is also advised that each company maintain a golden board to be tested before and after C&As and PMs. Verifying the same number of defects provides a simple way to check that the system is working properly, and that maintenance procedures were properly performed.

Training, program auditing, maintenance and feedback loops are all very important to ensure maximum value from the test/inspection tool. Other areas also must be addressed to ensure a proper implementation specific to a given situation. A checklist should be developed that identifies the critical components of the test/inspection tool, along with a plan of how/when they will be monitored. 

Barbara Koczera is a former Agilent engineer and founder of Koczera Consulting, LLC (koczeraconsulting.com); barb@koczeraconsulting.com.

Drilling may be behind ‘false’ gaps.

The microsection in Figure 1 was taken from the knee of a plated through-hole during board examination. The section shows a gap between the edge of the original copper foil and the through-hole copper plating. This is not a crack in the plating, however. Rather, there is resin smear on the copper foil edge, and the copper plating to the surface of the board and into the through-hole has simply covered the resin, giving the appearance of a crack or the copper folding.

As resin smear is the most likely cause on the edge of the copper foil, a full review of the steps during and after drilling needs to be examined. Test sections taken after panel plating should be considered to demonstrate any improvements in the copper plating adhesion to the edge of the foil. Reference should be made to IPC-A-600 or internal company procurement standards. 

Dr. Davide Di Maio is with the National Physical Laboratory Industry and Innovation division (npl.co.uk); defectsdatabase@npl.co.uk.