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

Printed Circuit Board Assembly & PCB Design Forum

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Which side of PCB should be prefered for screening...

shvetangcd

#17625

Which side of PCB should be prefered for screening... | 18 September, 2001

Hi Jeff,

We are having solder-short problems for BGA, QFP, and some other fine pitch parts while using DEK screen printers. The PCB is processed on the second half of the line for screening the side with BGA, QFP, and other fine pitch parts. Which means that before the PCB arrives to the DEK for these parts, it has already passed through the reflow oven of first half of the line for the other side of PCB (mostly with passive parts).

Is it possible to eliminate the solder-short probems for these parts by processing the PCB first with BGA, QFP, and other fine pitch parts and then the other side mostly with passive parts?

Thanks.

Shvetang.

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Jeff Schake

#17640

Which side of PCB should be prefered for screening... | 18 September, 2001

Why do you think that assembling the BGA, QFP, and fine pitch parts will solve your bridging problem? I would think doing this may cause you more grief by having these more massive components being reflowed upside down fall off during the second pass assembly. The passives are better suited for multiple reflow passes due to their smaller mass and better tolerance to withstand multiple reflow cycles compared to the BGAs, QFP, and fine pitch parts.

What is the earliest stage of the assembly process that are you seeing these shorting problems?

Scenario 1: After printing? In this case some questions to ask yourself are: a. Is the stencil designed correctly? b. Is the stencil damaged? c. Are the squeegees damaged? d. Is the solder paste rheology and formulation appropriate? e. Is the machine calibrated and operating properly? f. Are suitable printing process parameters being used? g. Is the board being fully supported with proper tooling? h. Is the under stencil wiping system working? i. When the board is printed the second time, is it still warm from the first reflow?

Scenario 2: After placement? In this case the placement pressure is probably too high, causing paste to squeeze out under the balls or leads and bridge.

Scenario 3: After reflow? Some things to check: a. Are you using nitrogen? b. Is the temperature ramp too aggressive? c. Is the solder paste you are using sensitive to hot slump?

-Jeff

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Michael Parker

#17641

Which side of PCB should be prefered for screening... | 18 September, 2001

... and don't forget proper stencil alignment. Invariably I find most bridging problems for fine pitch parts, etc. is caused by lousy alignment. The alignment must be verified under magnification, using the naked eye is not good enough.

Use at least 15x power. Preferably 50x or greater for pads under 15 mils width. Any paste slumping off the pad and into the solder dam of the mask is a potential bridging problem. Check for X, Y axis and also theta (rotation). Most people forget theta because it takes time to get it right. Just remember- it takes more time to do rework than to adjust the stencil alignment.

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Reflow Oven

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