As many through-hole components are being replaced by their surface mount counterparts, printed circuit boards (PCBs) are still being designed with both technologies. Manufacturers are pressed with process issues to build these PCBAs (printed circuit board assemblies). When designers are applying mixed technology on PCBs, the challenge is to determine the optimal method manufacturers can use to deal with the populated boards. One piece of equipment manufacturers may want to invest in is a selective solder machine.

Some of the selective soldering methods are selective wave soldering [1], selective fountain soldering [1], programmable selective soldering [1], miniwave soldering [2], laser soldering [2], round-table soldering [2], and hot-gas soldering [2]. The advantages and disadvantages of each method are described below.

The Selective wave soldering process is similar to that of a regular wave solder machine, where the board is soldered from the underside of the PCBA (printed circuit board assembly). Using this process is one way a manufacturer can deal with mixed technology on a PCBA if they already have a wave soldering machine. The process involves using fixtures that are specific to the PCBA, so the cost and storage of the fixtures are a consideration for the manufacturer.
Selective fountain soldering is also performed from the underside of the PCBA, and is a capability for those who would like to work with the same process as wave soldering. A manufacturer may consider this equipment for the rework or additional soldering of multiple through-hole components. As with selective wave soldering, fixture cost and storage add to the cost of purchasing this type of machine. The PCB designer will need to create a manufacturable PCB that will accomodate the process and capabilities of the machine. Additional operator time may add to the cost of operation when a semiautomatic machine is considered.

Programmable selective soldering is a growing process that deals with mixed technology on PCBAs. In this type of soldering, the build process is flexible, hence the manufacturing is likely to increase. A benefit of these machines is that they do not require fixtures. However, the programming and training of the operator increases set up time. It is important to realize that cycle times will vary by machine and significantly affect production costs. The acquisition costs of these machines are higher than those utilized by the previous two methods.

Selective miniwave soldering falls into two basic variations: robot handling [2] and pass-through systems [2]. The robot handling system is flexible and is customized to the buyer with upgradeable options. The pass-through system is not as flexible as the robot handling system, but offers higher volume with shorter cycle times during production [2].
Selective laser soldering is a desirable process where the machine can solder areas that are difficult to access. The machine can solder temperature-sensitive components, and is repeatable. The power to the laser can be adjusted to suit the requirements of individual tasks. Hot-gas soldering can be an alternative to laser soldering when using temperature sensitive components.

Round-table selective soldering can be configured to the needs of the customer. Up to six stations can be implemented in the machine to perform the various processes needed during the production run. Its flexibility is what may attract buyers to this machine.

Taking into account the variety and flexibility of available machines, users should be able to locate one that suits their individual needs. The cost of the machine is a consideration when deciding to improve the manufacturing process control of the PCBA with mixed technologies. Ultimately, the reliability of the process will most likely increase with the implementation of a selective soldering machine. Future machines will most likely be more versatile due to the trend towards lead-free manufacturing.

The EMPF utilizes the Beamworks Spark 100 to satisfy its selective soldering needs. If you would like a demonstration of this machine, please contact the author of this article or call the helpline at (610) 362-1320.

References:
1. "Selective Soldering", Robert Rowland, SMT Magazine, March, 2002.

2."Automated Selective Soldering of Mixed Technology PCBs", Robert Rowland, SMT Magazine, May 2002.