On 01:35 PM 3/03/2003, =?big5?B?Ikx1bywgWXUtTWluZyAow7mlya/6IElBQy1OKSI=?= said:

Dear All,

I always set the polygon connect to pads directly, and it always work fine. But now, we found there are components stand after reflow, some contain pads connect to polygon, some not. I wonder if this is caused by the polygon connet style or any other reason. So my question is:

1, what are the key points to cause components stand after reflow?
2, When will components-stand always occur during reflow process? Preheat or cool.
3, How to slove this problem?

I am looking forward to your constructive suggestions, any input would be great appreciated!

Tomb stoning is caused (I think) by differential reflow of the solder paste - the two pads melt and solidify at different times. When one pad is still melted the other may be solidifying and the surface tension can pull the component upright. This can be caused by lots of things, including footprint design (too much/too little paste), pad connections to planes and tracks, reflow oven temperature profiles

Direct connections of planes to surface mount parts is something I avoid if at all possible for very much this reason. You should try, if possible, to keep the copper load pretty even on both sides of a small component. I reserve this only for high freq RF boards (UHF and greater usually).

I have not seen tomb stoning for many years now - even from some pretty low end assemblers doing boards that we have not specifically looked at for tombstoning potential. This is something you should be discussing with your assembler or production staff (if inhouse assembly).

Look on the web I am sure there would be stuff about this. If you know a good assembler talk to them.

Ian Wilson

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