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QFP Defect

#7052

QFP Defect | 25 June, 2001

Hello,

I have a board that has been giving us problems for some time now. It is fairly well populated, mostly with IC's and SOT's on the topside. There is a Zilinx QFP160. This component does not reflow well. All of the other components look OK. A thermal profile of the component leads shows adequate preheat, ramp, soak and spike times and temperatures. The prints are checked for deposition measurements prior to placement. Post reflow, the solder on the pads for the QFP has migrated to the leads of the device leaving a dewetted appearance to the pad. In some cases, the solder has retreated and left no fillet on the toe of the lead giving the appearance of an insufficiant solder joint. If the lead is fluxed and hit with an iron, the solder wicks back out and leaves a perfect looking solder joint. Any ideas or comments regarding this situation would be greatly appreciated.

Thanks In Advance

Christopher Lampron

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Dave G

#7054

QFP Defect | 25 June, 2001

What type of board finish ?

I've seen this same defect with QFP's on some of our HASL boards. Best I can tell either the PCB's have poor solderability or uneven pad heights due to poor solder leveling at the PCB fab. Newer PCB's seem to yield better results than older ones off the shelf. I'm still looking for the root cause. We've tried changing the profile a little without much improvement. (Ran a little shorter soak - thought we might be boiling off the flux too early.)

We don't seem to have this problem on Nickel Gold PCB's that we run using the same P/N-Date code QFP.

Sorry not much help here, I'm looking for an answer to the same question.

DG

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Travis Slaughter

#7061

QFP Defect | 25 June, 2001

Do you by chance have a copper layer soaking the heat away from the pads? If you do or even if you don�t you may want to try bumping your bottom heaters up, try bringing the heat through the PCB. This may help pull the solder down to the pads. This might help I would think its worth a try.

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ianchan

#7066

QFP Defect | 25 June, 2001

Hi mate,

no complete info given from what i see, so just some humble comments, mayhaps you review :

1) PCB pad finishing - Copper+nickle plated? coz if so, nickle is a KILLER to good solder wetting. what we did back here, was to HAL coat the nickle, that solved our problems of 100% SOIC touchup solder joints. (nickle has good knock-resistance & handling durability, but is lousy for Sn63/Pb37 soldering process, unless you guys can go for Sn96/Ag4?)

1a) PS: Ni+Au pads are good, we moved to that too....

2) Agree with possible QFP legs non-coplaner, and some leads possible to have little contact to the pads during reflow. Hmmm.....check it out?

3) Any large Cu "land" areas surrounding the QFP region, check if the thermocouple measurements have large deviants in deg-C for QFP vs Cu land areas. If so, means Cu lands are dissipating the heat away from your QFP region? (then again you say other components ok?)

4) Lastly, do you pads have via holes next/beneath the pads? coz via holes put there for faster rate of heat dissipation by the designer, can cause solder "drainage", that results in "insufficent solder" defects.

Good Luck in your investigations, hope you can share findings with us :)

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#7068

QFP Defect | 25 June, 2001

If your solder paste has forsaken the pads and gone to all the work of climbing the leads, it�s because the leads are much warmer than the pads. Some like it hot!!! We talked about this on SMTnet within the past 6 months. Please check the archives for background and then get back to us.

When you say "A thermal profile of the component leads shows adequate preheat, ramp, soak and spike times and temperatures." Did you profile from the pads [not leads] on this problem component that doesn�t solder well?

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Chris Lampron

#7197

QFP Defect | 9 July, 2001

Hi Dave,

Yes it is HASL Solderability of the pads was my first hunch but the pad reflows fine when an iron is taken to it. Im will look into it.

Thanks for your input

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#7208

QFP Defect | 10 July, 2001

Hello Dave, Sorry it took so long to reply. When we did our profile we did attach the thermocouples to the lead foot right above the pad. I suppose that the leads could have been hotter than the PCB pads although, other areas that were profiled were on the pads and were within the paste specification. Now that I think about it, there is more Cu around the QFP due to the number of terminations. This could contribute to a lower board temperature at reflow. I will be running some more of this product this week and I will look into it.

Thanks for your input.

Christopher Lampron

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#7215

QFP Defect | 11 July, 2001

Hi Ian, Answers to your questions:

1) PCB Pad Finishing- HASL 63/37

2)Checked a sample lot of parts. Coplanarity did not seem to be an issue. Some of the parts checked still exibited this condition after reflow.

3) There are no large ground planes or thermal mass in near proximity to the component although, the fact that it is a fine pitch device means more copper per sq inch in this area. I will profile the component leads as well as the PCB surface near the component in question.

4) All of the nearby via's are tented. We performed an X-Ray to be sure that that was not the problem.

Thanks for your input. I will be rechecking the thermal profile this week. I will let you know my findings.

Thanks

Chris

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#7216

QFP Defect | 11 July, 2001

Hi Chris,

Dave F is absolutely correct in his assunption that the leads are hotter than the pads. You must profile from the pads to see the exact temperature that they are reaching. If the entire board reflows properly as you described, don't get caught up in the PCB layer, pad design and metal content, this will only complicate a simple resolution. It appears to me you are utilizing a tent type profile or a new brunswick profile with a short soak time. If you find your pads to be colder than the leads of the QFP, try increasing your soak time to allow the pads to become close in temp before spiking at reflow. Another thing to look at is the temp in which you are soaking. Is it up around 160-170 or our you on the low end around 130-150? This will make a difference when it comes time to enter the spike zones..

Regards, G

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