Before You Change, Ask! Print E-mail
Written by Lenora Toscano   
Thursday, 03 June 2010 21:16

Why that assembly problem fix might wreak havoc upstream.

Assembly has been on my mind constantly the past few weeks. I suppose assembly should always be in the forefront of my thoughts, considering I manage a Final Finish unit, but for the past few years, my thoughts have drifted to other aspects of product performance throughout the stages of the printed circuit board’s life. Lately, however, it seems the day-to-day questions I field center on the assembly process.

There were many times in my tenure when I stood next to the assembly line to see its  joys and heartaches. I remember the first time our immersion silver surface finish was assembled on a full production line. I was a new researcher, fresh out of college, in awe of a pick-and-place machine. I remember flying to North Carolina because a customer could not get the immersion tin to pass second-side assembly; lesson learned on required pure tin thickness. There were times when I watched a board get printed and saw it come off with perfect solder joints. Sadly, there also were times when I pulled a board from a reflow oven and looked at the horror of microvoids under an x-ray unit the size of my first apartment.

When you work on surface finishes, you constantly ask yourself, What is the desired functional performance? Selfishly, I think the task is much easier for those who develop the processes upstream. Those who develop a desmear line, for instance, do not have to take into consideration every step before and after their process, as I do. With surface finishing, everything comes into play. These factors range from the copper foil treatment to the soldermask preclean used. The assembly characteristics of each surface finish are not just a sum of the final finish process. It is actually a sum of the entire board manufacture and, in many cases, with the addition of the assembly conditions itself.

Ask yourself, Will any metal be left exposed in the electronics' final destination? What signal frequency will this board experience? Does it need to be wire-bonded? Is environmental resistance required? Sure, we can address all these issues separately. The challenge becomes greater when the final product encompasses all. By the way, every surface created also needs to have incredible solderability. Try explaining that to a non-final finish researcher. They throw up their hands.

Yes, there is a final finish for everyone, and no single surface finish will fill all needs. I see that as a great challenge, but one to accept and enjoy. Not only does it allow me to have a career, it keeps life interesting. (Albeit sometimes too interesting.)

As a chemical supplier, it seems our bigger challenges come after assembly is completed, but are always a result of what transpired upstream. “It went in fine, but then comes out discolored.” The dreaded, “The component fell off.” The ever popular, “The first side soldered great; the second won’t wet at all.” I have to ask, What happened to these few panels that differ from the thousands, probably millions, of boards prior that had no issues?

Something changed upstream, whether the operator on third shift fell asleep during a process sequence, or the design engineer decided they could change a pad from a square to home plate. Every aspect makes a difference.

The point is that I think more than twice before making any recommendations for change. What will be the effect of said fabricator switching from mechanical soldermask preparation to chemical cleaning? I know the soldermask guys never wonder about how that will affect the wetting of the final finish, or even appearance; it is not their immediate task. How will printing a varied aperture affect the solder joint integrity? How do additives in the electrolytic copper affect the coating thickness uniformity and appearance?

We need to use the resources around us and come together as a team. My resources for surface finish research and development start with my immediate team, and span from coworkers in other parts of the world, my customers, my customers’ customers, my peers in this industry, my competitors, their customers, the industry groups, test houses; the list goes on and on. We all need to work as a team in some aspect. Everything each one does affects the other, and we are constantly learning from each other.

Ask the questions before you make the change; ask many questions of different people in different aspects of the product design and end use. Run the tests before the final decision has been made. Ask others to run the tests as well.

Lenora Toscano is final finish product manager at MacDermid (macdermid.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it . Her column appears bimonthly.

Last Updated on Tuesday, 17 August 2010 18:34
 

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