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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Misalignment during reflow

Thomas

#8609

Misalignment during reflow | 9 November, 1999

Hello everybody,

Has anyone come across PLCC-44 & above misalignment after reflow. I tried to solve the problem by eliminating factors one by one until at one stage, I open up the oven when the board is half way through the oven (in preheat zone)and found the PLCC already misaligned. I saw slumping of the solder paste and decided to lower the first 4 preheat zone temp by 10 to 50 degree C. This eliminated the problem. As this product had been running for quite some time w/o such problem, I suspect the root cause is not the preheat profile but the solder paste involved. I sent the solder paste back to the supplier for slump test but the result is negative (within their spec of > 10% slump). I am finding out their test std used but I think those lab test is different from real reflow as the heat transfer rate of reflow oven is much higher.

Any one care to share your views?

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Charley Qin

#8610

Re: Misalignment during reflow | 11 November, 1999

Hi, TOM

Did you check your oven's chain movement? Is it stable?

Good luck.

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#8611

Re: Misalignment during reflow | 11 November, 1999

Thomas: From what you're telling us, you seem to be thinking correctly:

1 Paste should not spread appreciably in preheat 2 Something has changed, because you've not had this problem, until recently

Two things to consider:

1 Has any thing changed in your oven? Blowers are wipped, profiles have stepped, etc 2 You may want to qualify your paste suppliers for the product you are manufacturing. Paste suppliers go stupid periodiclly and it might be good to have base-line measures of paste performance. Below is a snip on slump measurement from our procedure.

4 Slump Test 4.1 Print paste on white ceramic substrate or microscope slides. Using the stencil described in paragraph 3 above. 4.2 Inspect the paste at X30 magnification. The shape of the deposit should be a "brick." Record the pitch that slumping became prevalent. 4.3 Bake the substrate at 150�C for 2 minutes. 4.4 Inspect the paste at X30 magnification. There should be no slumping. Record the pitch that slumping became prevalent.

Stencil description from 3:

3.1 Obtain a stencil with 15 to 20 rows of a series of 0.025 by 0.050 inch apertures spaced 0.050, 0.040, 0.025, 0.015, 0.010, 0.010, 0.025, 0.040, 0.050 inches apart. See Appendix 1. Source of supply: Metal Etching Technology.

Good luck

Dave F

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