Once upon a time, all printed circuit board (PCB) assemblies were designed with the same basic construction. The “primary side” was the installation side for through-hole components and contained surface mount technology (SMT) components while the “secondary side” had exposed pins for through-hole components and might have some SMT components. The process engineer was faced with a very basic assessment when determining the correct assembly method. Most assemblies were processed in the same way. The secondary side had glue dots applied, SMT components installed, and the glue was cured (if SMT components were present). Then, the primary side had solder paste applied, SMT components installed, and the solder was reflowed. This was followed with through-hole installation and wave soldering. The biggest concern for process engineers dealing with the through-hole components was controlling solder shorts and ensuring design engineering understood how to optimize the design for wave soldering.

Unfortunately, the “good old days” of straight-forward mixed-technology design is mostly a thing of the past. Today, electronic assemblies are more complicated and have smaller and smaller form factors. This forces both SMT and through-hole components to take residence on both sides of an assembly. Many designs are highly integrated, with the vast majority of parts taking SMT form. However, there are some parts that don’t lend themselves to an SMT package, such as beepers, switches, and user access connectors. Multi-image panels are utilized for many designs in order to maximize the efficiency of the PCB construction and assembly. This can create a situation where it is impossible to control co-planarity of the entire panel due to its overall width. All of these factors are driving the assembly industry to adopting “selective soldering” processes.

Selective soldering refers to the direct application of solder to specific areas of a PCB to form through-hole solder joints, rather than the “all or nothing” approach used in wave soldering. There are a variety of processes that fall under the heading of selective soldering. These include robots that mimic hand soldering processes (a programmable solder iron and solder wire feeder), robots that use lasers and solder wire to form solder joints, programmable machines that precisely solder by moving a mini-wave solder pot or assembly to specific locations, and masking pallets that expose only specific locations of the board to a standard solder wave.

Palletized selective soldering is an excellent way to introduce more complicated product designs that stray from the standard mixed technology design, with little investment in time and money. In most cases, the wave solder equipment is already part of the existing assembly process. The process knowledge may already exist with the personnel, equipment maintenance procedures, and raw material supply already present. Masking pallets typically cost less than $1000 each and can be sourced from a number of commercial suppliers, or designed and fabricated in-house. Even with a low percentage of assemblies that require selective solder, wave solder equipment can still be utilized for standard designs. Processing pallets over a solder wave require only minor adjustments that can be changed in a matter of minutes.

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 ten 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 allow sealing with the pallet.

The reasons for requiring a selective solder pallet can be varied. In some instances, a pallet may be required due to SMT components on the secondary side that can not 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 components in SMT form, 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 allow for 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, the use of masking pallets can allow 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 the standard conditions. Most importantly, the pre-heat temperatures will 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, just as is true for a standard wave solder application. Wave height settings will 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 allows 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 allow a manufacturer to capitalize on existing equipment and process knowledge, while increasing product complexity and capabilities. For more information on selective soldering with pallets, please contact Ken Friedman at 610.362.1200, extension 279 or via email at kfriedman@aciusa.org.