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pc board defects

Ken Bliss


pc board defects | 13 September, 2002

I recently saw an ad in a major trade magazine that stated:

�about 60%of all printed circuit board manufacturing defects are due to stencil printing�

I would like to hear comments from people that are running SMT lines, is this fact and if it is, what are the actual defects being found and what is been found to cause them

bad solder paste

dirty stencil

misalignment of stencil and board


Thanks in advance for taking the time to comment

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pc board defects | 13 September, 2002

I can't give you itemized values for each defect category because we don't ask operators to distinguish between them, and we haven't done any serious research to do so either.

I do wonder this, though. I read data somewhere from 14 years ago stating something similar. Fourteen years later the improvements in printer and stencil design hasn't reduced that to less than half? I don't believe it.

We reduced our standard pitch defect numbers (that we attributed directly to print problems) by about 25%, our fine pitch stuff about 75% when we got a new printer. Some of the reduction was based on repeatability of the printer, some on vision finding the defects, and some on reducing the requirement for operator intervention. The point is, I'll bet that 60% figure is invalid these days EXCEPT for those that still are struggling with the unenviable task of setting up a worn out clamshell to print 2400 pins worth of 20 mil pitch on a 10"x20" dsp board.

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pc board defects | 14 September, 2002

We need to remember here that as printer manufacturers were enhancing their product lines, miniaturization was quickly being developed in the background. In the last 2 years I was involved in developing and conducting a D.O.E./F.M.E.A in determining Print CPK. An internal Team was identified will the first task of identifying high level root cause, then a fault tree diagram was developed to Indentify print characteristics and variables associated with the process. A very detailed set of print tests were developed and conducted. The long and the short of it is we determined that the printers themselves are extrememly accurate and repeatable, mechanically, print to print etc., it is ALL other variables that affect printing performance. Our tests indicated many short falls associated with: Operator to Operator repeatablity Paste relax and recovery (Stencil life) Under wipe solvents Handling of boards Cyber Centry coverage and equipments inability to inspect 100% of sites. (escape factors) Stencil Design Stencil cleaning between runs The highest attributor was clogging apertures in fine pitch, resistor networks, and BGA, Micro BGA areas

Obviously there was a lot more "Meat and Potatoes" to this study.. But, yes...60-70% of SMT defects are print related. As the component manufacturers continue to develop smaller and smaller footprints the challenge will remain in keeping up with technologies to support accurate and repeatable paste deposition, especially on High Mix boards with an average of 45,000+ opportunites. I remember a well known trainer and industry expert (Phil Z.) about 5 years ago stating that 95% of all SMT related defects are printer related. So we have come a long way. Scott

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Ken Bliss


pc board defects | 14 September, 2002

Hi Scott Thanks for your comments, you stated "The highest attributer was clogging apertures in fine pitch, resistor networks, and BGA, Micro BGA areas"

was this do just to the size of the aperture or bad or old paste or debris from inadequate cleaning?


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pc board defects | 15 September, 2002

That 60+% print defects number has been bantied around for so long in the electronic assembly business for so long that people believe that it's true. One starting point for this myth and lore is: "Where Quality Is Lost On SMT Boards" C Mangin, 'Circuits Assembly', 2/91 breakes-down process defects as follows: * Solder paste screening: 63.8% * Component placement: 15.3% * Reflow solder and cleaning: 15.2% * Incoming components: 5.7%

The industry is doing things differently now.

We has a similar discussion on SMTnet. Look here:

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pc board defects | 15 September, 2002

My 2 cents...

Screen printing may very well have a leg up on deciding SMT defects... however if you pay attention to your stencil (aperture size ratio etc..) and inspect your prints... I believe its a much lesser number... (paste MFG also plays a role(IE slump, tact etc...) I currently see more defects determined by component packaging such a tubed components or Trayed component which are mis-handled.. also tombstoning related to oxidized/contaminated components... I think the real bottom line is how much you pay attention to your process... 1.. Inspect all prints after printing 2.. Inspect all tubed/trayed parts for bent legs before you use them. 3.. Inspect all boards BEFORE reflow. 4.. People also play a huge part.... reward those who care or pay attention to detail ,.. reassign those who dont...

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