Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.




Paste | 18 April, 2004

Hi, I am a Quality person (Metalwork fabrication and System assembly)but have not had any experience of SMT before. I am trying to put some SPC in place for reflow and others and have come across something I do not understand. Our current SPC Chart controls humidity between 10 & 60 percent (Is a mistake as they have been using specification limits and not UCL and LCL on charts). I looked at the paste specifications and it says between 30 and 60 % as the specification. Our machine is consistenly at 15 to 25%. I have seen lots of information which tells me how bad it is too have too much moisture but what about too little. Does it even matter in the reflow as long as we have been storing it well before hand and do not leave it standing on stencils for production. Apologies if this does not make much sense but have a total of 3 weeks experience in this area and am struggling to understand the controls I should have.

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Paste | 19 April, 2004

I have seen problems when printing below 18% with our paste. We use a swamp cooler to dump moisture back into the room. It usually get to about 28% and the printing issue disappear.


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Paste | 19 April, 2004

Rob: We're never heard of paste related problems caused by humidity that is too low. Low humidity is very expensive to control. So, when you think about all the assembly used to be done in the Bay Area. You'd think there would be more talk about it if it was a BIG problem.

Commenting on your controling "humidity between 10 & 60 percent": Some pastes are 10-60% RH. It's possible that control was appropriately instituted for a different paste, but was never changed when a new paste was added.

Sparky: What specific problems have you experienced with low humidity?

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Paste | 20 April, 2004

Hey Dave,

We have had problems with low RH. In a very short amount of time, the paste dries out and starts to stick to the squeege blades. We would see an increase in skips and some clogged appertures. This has only happened when the RH was 15% or below and only happened in the DEK's.



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Paste | 22 April, 2004

Thanks Chris. So, what do you figure is the reason that this only happens on DEK printers?

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