An optimized SMT assembly process typically provides a yield of nearly 100%. Technology advancements—from the solder paste printing process, SPI, and parts placement, to reflow and wave soldering and AOI—have pushed the efficiency and accuracy of these steps in the SMT process such that a board assembly should be perfect at the end of the line.
Still, EMS providers continually face the need to rework and repair PCBAs even after dialing in the perfect set-up; there will always be that insufficient solder, or excessive solder, or missing component on the board, among other issues. Chief among them is the continuous trend toward miniaturization in the industry—the ever-shrinking component sizes being placed and soldered onto boards with finer and finer pitch and spacing—which is putting a lot of pressure even in the rework and repair of such boards.
Also, with reliability being one of the top customer requirements, touching boards should be avoided because it increases the chances for damage, such as flexure or shearing of parts with the soldering iron on the back or front side, according to Gary Freedman, of Colab Engineering. He notes that every time a board passes through a repair cycle, that board will be of lesser reliability.
You need rework—there will always be a need for rework—but the more you do rework, the more touches a PCBA receives, the higher the chance its reliability decreases. What a Catch-22 situation.
For this month’s issue of SMT Magazine, we talked with BEST Inc.’s Dan Patten and Laura Ripoli, Circuit Technology Center’s Andy Price, and Freedman to find out more about the critical challenges in rework and repair of PCB assemblies, and which strategies to implement to improve the process and ensure the reliability of the boards.
Interestingly, one of the things they pointed out is the skills of the operators or technicians doing the rework. In the words of Andy Price, they should have "a tremendous amount of experience, hand skills, the ability to work using magnification for hours on end, knowledge, and patience." The skillset, according to him, is just not on the market, so you must find somebody with the right mindset and right capability to do the job. Once they are inside your doors, continuous training is required.
To read the full version of this article, which appeared in the September 2017 issue of SMT Magazine, click here.