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NXP QFN Solderability Wetting Problems

Views: 265


Tom

#78647

NXP QFN Solderability Wetting Problems, Kester | 9 August, 2017

We're having problems with Kester pastes (NP545 pb free and EP265 60/40 no clean pastes.) All of our other components reflow just fine with either paste, but for some reason this QFN chip doesn't wick to the QFN's pads well. It seems like some type of problem with the paste flux in my opinion. We matched the data sheet profiles and then tried dozens and dozens of adjustments with temperatures and speeds with no real satisfactory or repeatable results.

What other paste manufacturers might we try to see if we can get better results on this chip from a different formulation?

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#78651

NXP QFN Solderability Wetting Problems, Kester | 10 August, 2017

How do you evaluate the soldering quality for those QFNs? X-Ray or AOI?

For some QFNs (and other bottom termination components) it's not supposed to have solder fillets on toes. It's also noted in IPC-A-610 (Rev.F 8.3.13). Take a look at your QFN chip's toes under magnifying lens, you will see why.

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Tom

#78656

NXP QFN Solderability Wetting Problems, Kester | 10 August, 2017

We are just using a stereo microscope at this time. When we first started out with these parts a year ago they filleted very well but with no apparent change they began having problems. While the boards do work, we are concerned about the long term reliability. We have seen some come back with this part overheating and no other apparent problems so we suspect solder voids ocurring in the field due to poor quality joints on this part.

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dwl

#78657

NXP QFN Solderability Wetting Problems, Kester | 10 August, 2017

The exposed metallization on the sides of the QFN probably aren't tinned and therefore not expected to have solder wetted to them. If previous parts formed nice toe filets, it might just be that those parts were fresher and the exposed metal did not have time to oxidize.

As for overheating in the field; Assuming its not a design defect, I would look to voiding on the center thermal relief pad. It might be worth having some of the failed boards X-rayed. I suspect you will see large voids which prevent heat from being conducted away from the part.

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#78693

NXP QFN Solderability Wetting Problems, Kester | 14 August, 2017

AIM, Indium , Alpha will be your top 3 choices. I did paste evaluation before with 4 paste manufacturers and Kester was my choice number 4. Profile according to the paste specs and observe results.

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