The Second Nature of Process Control Print E-mail
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Written by Lenora Toscano   
Thursday, 01 April 2010 00:00

Our newest columnist seeks to expose the synergy of fabrication, assembly and end-use.

Hello! I’d like to take a few moments to introduce myself, though I feel like I have known most of you for years. I have the opportunity to write this column as a result of my predecessor, John Swanson, moving on to smaller and more expensive things. John has taken the task of expanding our business to include electronic packaging. After 10 fast-paced years in the industry, I now find myself managing a business unit.

A little about me. I love yoga. It never allows my mind to travel to the pressures or issues encountered in life. But what I enjoy most is that I usually feel like a superhero after the class is completed. This last time, though, there was a new instructor, who began the class by saying, “Tonight we are going to concentrate on the basics.” Great, I thought. How can I reach superhero strength if we are going back to the beginning?

Yet, I left an extremely sore and beaten woman. Not only was I a better person than the one who entered, but I had my idea for this column: How can we expect greatness if we do not pay attention to the basics?

It’s cliché, but building a strong foundation paves the way for success in everything we do. Consider process control. Again, I know part of you is cringing, but there is another side that remembers the last three product issues experienced came down to poor process control. Ensuring good control will result in the best possible products we can make.

Did you know that when the board goes through the reflow oven, if vias are not properly plugged with soldermask, the mask cracks, leaving exposed copper? As in Figure 1, the copper found through the crack has not seen any protective coating and becomes a breeding ground for corrosion. If soldermask registration is off on a pad, and the solder cannot collapse properly, the resultant joint is more susceptible to thermal fatigue.

Imagine a BGA pad that has one side soldermask-defined, as a result of misalignment, and the other metal-defined. The solder collapse will not be uniform. During reflow, the soldermask will expand on the one side, creating pressure on the sphere, which acts as a lever. This can promote joint cracking.

Inadequate copper preparation will result in nonuniform coverage of an OSP coating. This is true for all surface finishes, though shortcuts often are taken at the cleaner and microetch steps of a process. If the copper surface has contaminants, the finish will not properly bond to the copper. This can manifest itself as nonuniform coverage, poor surface coating adhesion, insufficient thickness, solder joint voiding, and even premature tarnishing.

Also true for any surface finish is the importance of coating thickness. Believe it or not, thickness specifications were put on the technical datasheets for a reason. The specified thickness is critical to the coating’s performance. Everyone has experienced an insufficiently plated tin deposit. After one reflow, all the pure tin had been quenched by intermetallic that laughed at you on the second assembly pass.

Focusing on process flow and routine analysis can eliminate many future quality issues. Did you know sending a board twice through Pb-free HASL can embrittle the soldermask to the point where the resultant product will have much worse creep corrosion than any thin immersion silver deposit? Did you know the first instance of soldermask interface attack on an immersion silver board was found as a result of a customer continuously dragging microetch into the following rinse, which did not have a sufficiently strong turnover? Basically, the PCBs were being double-etched, and then put directly into the pre-dip bath.

On a positive note, running the right process controls can make a better product. Choosing the proper equipment for immersion tin can minimize solution air exposure and extend bath life. Premature tin oxidation and thiourea decomposition can be greatly reduced. Using the right pre-clean, including a well-maintained microetch, can enhance the OSP and immersion silver coating quality, which in both cases enhances solderability.

Maybe you do realize all this. But did you realize how much research at a chemical supplier goes into widening an operating window to accommodate the “what ifs” in fabrication? It is amazing what is asked of the surface finish on a day-to-day basis.

I don’t want this to be a finger-pointing exercise. I want to provoke thoughts on how to make process control second nature. I want to expose the synergy of the fabrication, assembly and end use performance. Every detail affects the next step and ultimately the final product.

When I started on this journey, I learned quickly that there was an application for each surface finish. They all have strengths and weaknesses. If processes are run according to specifications provided, and the same can be said for the processes around them, the resultant product will be superior. There are many instances where a coating outperforms the expectations of the chemical supplier, fabricator assembler and end-user. Imagine a surface finish produced as a result of everything run under optimal conditions: superhero status.

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 .

Last Updated on Tuesday, 17 August 2010 13:29
 

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