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This is a 2 x 2 mm chip with 12 no lead contact points. Half of the time I get it out of the reflow oven, it's twisted like in the image below.

I am using a stencil in order to lay the paste down and then tweezers for placing the little chips. There is no fan in the oven and I let the boards drop to < 170 C (leaded solder paste) before pulling them out. There are also some even smaller 0603 parts which never twist / tombstone.

How do they become crooked, and what can I do in order to avoid it ?

enter image description here

JYelton
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kellogs
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    How long are those pads?! I cannot be sure, but there is a good chance it is due to that. If the chip is even slightly offset, or when pushing the chip down, the solder paste is displaced slightly, it could cause the chip to end up like that – MCG Jul 12 '19 at 14:48
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    Have you double-checked the spacing of the pads and the spacing of the pads on the part? Is there a reason your solder mask is so long? It should only need to be about 3-4x the width of the pad. – TimWescott Jul 12 '19 at 15:00
  • @MCG how is the crookedness due to long pads ? It would be awesome if that was the reason; easy fix ;). As for the paste smearing, I don't have a pick and place machine, not sure what I could do about it. – kellogs Jul 12 '19 at 15:07
  • @TimWescott yes, this is an old design, just not meant for the stencil. Hence the long pads (i.e. I was applying paste by syringe until not long ago). Noted the length for my next design! – kellogs Jul 12 '19 at 15:09
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    I also believe the long pads are causing this. In the oven, all the paste doesn't melt at the same time. Those long pads that flow first have a lot of surface tension and can pull the chip out of alignment before the rest of the paste has a chance to flow. – evildemonic Jul 12 '19 at 15:19
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    You can apply some peelable soldermask to see if it helps. After using the stenicl, wipe off the excess as close to the chip as you dare, then apply the temporary soldermask before placing the chip down. – evildemonic Jul 12 '19 at 15:21
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    You can probably fix these one at a time with a hot air rework and bumping them back into place with the tweezer tip while hot. But to really solve the problem you need to move the solder mask back to the pad, and take care to make sure that the footprint is "balanced" - even large areas of copper nearby but not actually in contact could potentially cause differential heating/cooling problems. – Chris Stratton Jul 12 '19 at 15:22
  • I wonder if kapton tape would survive as a temporary mask... – Chris Stratton Jul 12 '19 at 15:22
  • @ChrisStratton: As Kapton is polyimide (as used in flex PCBs) I would think it would survive. – Peter Smith Jul 12 '19 at 15:29
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    @kellogs please show the following: 1) PCB artwork with pastemask layer and pad. 2) did you use a paste stencil? 3) What kind of paste? 4) Is there a pad underneath the DFN? – Voltage Spike Jul 12 '19 at 15:48
  • I vote for a an underneath pad – carloc Jul 12 '19 at 20:13
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    @evildemonic I suggest re-posting your comment as an answer. – JYelton Jul 12 '19 at 22:24

1 Answers1

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This is most likely caused by the long exposed pads.

In the oven, all the paste doesn't melt at the same time. Those long pads that flow first have an enormous amount of surface tension and can pull the chip out of alignment before the rest of the paste has a chance to flow.

You can try to fix this with a temporary peelable soldermask:

enter image description here

After applying your paste with the stencil, wipe off the excess as close to the chip as you dare (perhaps 1 mm). Apply the soldermask over the pads you wiped free of paste, then put your chip down.

As Chris Stratton meantioned in the comments, kapton tape could also work. Depending on the type it may leave a residue, but you can probably just leave it applied.

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evildemonic
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  • Not having any kapton tape around I have tried it with regular paint. It does the job, I wonder if it causes any long term damage to the boards though. – kellogs Jul 14 '19 at 15:03