SMT assembly is an optimization process. There is no single stencil printing process for all PWB designs. The stencil printing parameters of stencil design, squeegee speed, snap off speed, stencil wipe frequency, and solder paste for assembling all PWBs will not be the same; just as there is no single reflow oven profile for all PWBs. Fortunately, most solder paste specifications give good boundaries for all of these parameters, but typically some trial and error experiments will be needed when assembling a new PWB design that is not similar to past assemblies.
The need for optimization is most obvious when trying to minimize defects. As an example, minimizing graping is often facilitated by using a ramp to peak reflow profile. However, the ramp to peak profile may acerbate voiding. See Figure 1.
Figure 1. The ramp to peak reflow profile may minimize graping, but acerbate voiding.
Thankfully your SMT soldering materials and equipment suppliers deal with these optimization issues on a daily basis. So if you are ever stuck with some challenging SMT assembly process, contact these solder materials and equipment experts first.
It was Wednesday evening and I had just finished a brief pitch on applications of SPC to a group of 20. I was followed by Jim Hall, who spoke of process mapping using SIPOC. So did these folks have solder paste under their fingernails, or wave solder flux stains on their shirts, or, perhaps, a solder preform or two stuck in their pant leg cuff?
No — none of these souls would have had any of this type of trace evidence of electronic assembly on their person. You see, they were all medical doctors and students at Harvard’s famed medical school (see image below). (I hope it is OK that the proud dad shares that my daughter Jessica is a colleague of these folks.)
Jim and I were presenting to the doctors, because they are interested in process optimization in the healthcare industry. The event was hosted by Dr. Andy Ellner. He is a professor and doctor at the medical school and is a focal point for these process improvement efforts. I was introduced to him in the summer of 2009 by Dartmouth’s new President Jim Kim.
In November 2009, Jim, our colleague Larry Parah, and I facilitated Andy’s team in dramatically improving the prescription refill process in Brigham and Women’s Hospital Clinic. Jim and I plan on working with Andy in similar efforts over the next year or two.
The most striking thing that Jim and I left with on Wednesday evening was how profoundly interested these doctors and students were in healthcare process optimization. The Q&A session lasted nearly an hour.
Ah, yes, would that our many colleagues in electronic assembly were as interested in optimizing their processes!