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March 18, 2007

Stencil printing foil life-cycle

In stencil printing process, is there any minimum/maximum number of passes for a 5 mil foil stainless stencil before it needs to be replaced to a new one?



E.R.

Experts Comments

This is a question that has been asked many many times for a long long time. The short answer is there is no answer.

There are many factors that influence how many print cycles a stencil will last.

The down stop and pressure of the squeegee blade on the stencil, the support tooling, how the stencil is stored cleaned, and handled, etc. will all influence the life of a stencil The vast majority of stencils get damaged or obsoleted long before they "wear out".

The most common form of damage is what is called "coining" where the force of the squeegee creates a "bend" in the stencil at the point the stencil is not supported by the support tooling. In all my years of operating and visiting SMT manufacturing processes I have never seen a stencil actually worn through.

If you are using Statistical Process Control (SPC) to monitor your printing process by measuring solder paste volume or solder paste height you may notice a drift in your height or volume measurement if the stencil is in fact wearing but I have never personally experienced this happening.

I have seen many stencils damaged and obsoleted. We cut out and discard the foil and send the frame back to the stencil supplier. I also do not know of any formal studies that have been done to quantify the life of a stencil. Any formal study would have to be done using particular printing process operating parameters, so what you would experience in your process would differ based on your particular operating parameters.

My philosophy has always been if in doubt replace the stencil.

For the relatively small cost of a stencil why risk crating process defects? A few defects created by a "bad" stencil will far out weigh the cost of a new stencil.

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Joe Belmonte
Principal Consultant
ITM Consulting
Mr. Belmonte has been a process engineer and process engineering manager in the electronic manufacturing industry for over 25 years, with experience in all aspects of electronic product assembly operations. He is well-known throughout Asia and SE Asia for both his process work and teaching engagements.

There are many variables that affect the life of the stencil foil, particularly set-up parameters including squeegee pressure and material. I have had foils endure more than a half a million print cycles but usually, the pattern is obsolete long before the stenicl is worn beyond use.

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Phil Zarrow
Principal Consultant
ITM Consulting
Mr. Zarrow has been involved with PCB assembly for more than thirty years. He is recognized for his expertise in troubleshooting SMT manufacturing and lead-free implementation. He has extensive hands-on experience with set-up and troubleshooting through-hole and SMT processes throughout the world.
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