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

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Skewed components

Steve

#6078

Skewed components | 24 April, 2001

We are experiencing a fair amount of skewed components, mostly 0805 resistors and capacitors, during the reflow process. We have pretty much ruled out the pick-n-place and have determined that it may be happening because of variables at the paste screening process. (However, The paste is not offset a noticable amount.)

My questions are, if paste is offset by just a small amount (as little as 5%), would it cause a part to skew? Also, if the volume varies from one side of a part to the other, would that cause a part to skew during reflow.

Thanks for your responses.

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CAL

#6079

Skewed components | 24 April, 2001

I would look at the pad to part ratio.Over sized pad geometry will give the component more room to move. I have also seen where the two pads that make up chip components are not symetrical nor complimentary.The pads should be equal in the X or Y direction depending on the component orientation.

Caldon W. Driscoll ACI USA 610-362-1200 cdriscoll@aciusa.org

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Casey

#6082

Skewed components | 24 April, 2001

We solved similar problems some time ago.

Check the volume dispensed or printed. Also check that the glue or solder paste has not expired and if in doubt use a new cartridge or change supplier.

Ensure the Z axis stroke and placement force is correct for the component. Let us know if you need more tips.

Casey

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#6084

Skewed components | 24 April, 2001

I prefer mine skewered with a remoulade, yum.

IPC-A-610 allows component side overhang to the lesser of either up to 25% of the component termination width or up to 25% of the width of the pad for Class 3 devices. Help us understand your problem.

How did you determine that the P&P was not causing the skewing and that the printer was causing the skewing?

Other questions to ponder are: 1 How well do your pads compare to common pad design guidelines, such as IPC-SM-782? 2 What satisfied you that this problem is not caused by handling? 3 What makes you certain that the blowers in your reflow oven are not causing the components to skew? 4 What do the board side of the terminations of the skewed components look like?

Responding to your questions: 1 I�m not sure what you mean by offset, but we like to keep the paste within 0.0625" of 0803 pads, when printing. 2 You are unclear about how much variation you allow between the pads of a component, but if the paste volumes are within 10% of the size of each other, I�d run with it.

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#6085

Skewed components | 25 April, 2001

Steve: Try to find something that�s common to all your skewed parts. We haven�t had much trouble with it except for some MELF diodes a couple of years ago which didn�t look well aligned but made no trouble with specs. Better suited pad design solved this problem as in the most cases I saw.

BTW, I would be glad if we could manage as little as 5% offset in paste print not to speak of the volume, still we get results that do not force us to elaborate things further.

Wolfgang

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Steve

#6100

Skewed components | 25 April, 2001

Most of the time, we have very few skewed parts so I feel comfortable about the processes in general. However once in a while, often enough to throw off the first pass yield, we will get a few boards with a lot of skewed parts. By skewed I mean that one end is on the pad and soldered, and the other end is completely off the pad.

Before reflow, we can see that the part has been placed correctly. But then after reflow, it is skewed. I don't believe that the offset paste is causing all of it though.

The fact that at times they are good, and other times they are skewed, leads me to believe that it is a paste volume issue. The ratios of aperture to pad may also be an issue, but not optimal so that the process window is small.

Sounds like I have a few things to check out.

Thanks for the input.

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