I was wondering if I could get some tips on how to reliably solder a TQFP-100. I am using a Kapton stencil to stencil to apply the solder paste. The issue that I am having is the when I reflow the solder paste, the chip doesn’t align properly on the pads. It is like the chip is shifting between the pads. Any suggestions? Thanks in advance!
Are you saying that the IC is moving during reflow? Or is the problem that the IC is not self centering itself during reflow? Or something else?mdhendri:
I was wondering if I could get some tips on how to reliably solder a TQFP-100. I am using a Kapton stencil to stencil to apply the solder paste. The issue that I am having is the when I reflow the solder paste, the chip doesn’t align properly on the pads. It is like the chip is shifting between the pads. Any suggestions? Thanks in advance!
I haven’t had problems with reflowing TQFPs of this size as long as they’re placed properly. However, it is important to get it placed properly before reflow.
I don’t have any problems drag-soldering QFP parts with my Metcal system.
NleahciM:
Are you saying that the IC is moving during reflow? Or is the problem that the IC is not self centering itself during reflow? Or something else?I haven’t had problems with reflowing TQFPs of this size as long as they’re placed properly. However, it is important to get it placed properly before reflow.
The issue is that the IC is not self centering which results in the pins in between the pads. When I put the solder paste on should it be discrete 3 dimensional rectangles on the pads or will the solder paste spread a little bit onto other pads? The solder paste seems to spread a little bit which makes a line of solder paste across the pads. I know that this is the issue of why it is not self centering but I can’t seem to prevent it. Any suggestions? The solder paste that I am using is the Lead-Free No Clean Solder Paste from Manncorp.
I’ve never tried any hand pasting of my own before, or reflowed in a toaster oven or skillet but I work in an assembly house where we regularly reflow, TQFP’s, QFN’s, and BGA’s. Here’s a few things to keep in mind when doing these fine pitch parts.
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Your paste needs to be DEAD ON. You said that there is already some bridging with the paste before reflow and this will not work well at all. Take your time with pasting and be especially careful that you don’t smear the paste when lifting off the stencil.
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Most lead-free pastes do not flow at all. This means it will not pull your part onto the pad, wherever it is, that’s where it’s staying. So your part placement needs to be DEAD ON as well. More expensive pastes containing silver and other elements will flow much better. But in general lead free is much more difficult to work with than leaded. I would consider switching if tolerable/possible.
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I don’t know what your reflow setup is, but temperature profile is important in reflow. Try to have a smooth, gradual warm up period to let the fluxes mix well with the solder. Also, check you “time above reflow” this is the amount of time the board spends above the reflow or melting temperature of the paste. It may be that this is not long enough to allow the paste to become molten and have time for the surface tension to pull your part into place
I hope this is at least somewhat accurate information… 
Let us know what you find out!
I recently noticed an issues with one of the TQFP packages in the Microchip library. The package is designed with metric spacing but (at least) one of the packages was drawn using the English equivalent units. Over the length of 10 pins, the rounding errors make the pads almost half a pin off. The picture below shows the error comparing packages designed on a metric basis to a package on an inch basis.
You may wish to verify the package you’re using was correctly drawn.
Try getting some proper solder (ie, containing lead)!
Even with 0.5mm pitch QFPs I’ve never had a problem with self-centering of parts, as long as the pins are approximately aligned before heating. It is quite normal that the solder paste will spread out when heated, but if you have used the correct amount it should reflow OK. If you use too much, the worst that should happen is you’ll get some solder bridges.
Larger parts often have trouble self centering. The larger the mass/total pad area ratio, the worse it'll be. So you may want to see if just carefully placing the component fixes the problem.mdhendri:
NleahciM:
Are you saying that the IC is moving during reflow? Or is the problem that the IC is not self centering itself during reflow? Or something else?I haven’t had problems with reflowing TQFPs of this size as long as they’re placed properly. However, it is important to get it placed properly before reflow.
The issue is that the IC is not self centering which results in the pins in between the pads. When I put the solder paste on should it be discrete 3 dimensional rectangles on the pads or will the solder paste spread a little bit onto other pads? The solder paste seems to spread a little bit which makes a line of solder paste across the pads. I know that this is the issue of why it is not self centering but I can’t seem to prevent it. Any suggestions? The solder paste that I am using is the Lead-Free No Clean Solder Paste from Manncorp.
In the past, I’ve just drawn a line of paste down all the pads and then carefully placed the IC. This works well except that I’ll always find a few shorts that need to be fixed with an iron and some flux.
Using the proper stencil and getting the placement just right would be idea - but if you aren’t using a stencil already then I’d start with just trying to get the placement dead on.
And, as was already said - use real solder rather than the RoHS stuff if you are able.
When using solder paste on larger chips, I err on the side of too little paste rather than too much. As said above, it’s important to get the solder in the right place, and I always make sure the part is exactly aligned by inspecting with magnification. Normally there is no shifting, possibly some unsoldered pins and some solder bridges. Easy enough to retouch with a soldering iron.
Of course, drag soldering with my Hakko soldering iron works well too.
With RoHS solder, you need to use lots of flux. And you don’t get much reworkability, the solder crystallizes after a couple of reflows. If you have big problems it’s often faster to get out the hot air tool and remove the chip, wick off any solder, and redo the job.