Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


qfp reflow problems

barry

#28098

qfp reflow problems | 15 April, 2004

Hi there all, we are having problems with 2 qfp devices. grainy poorly reflowed solder joints. One 100 pin .5mm pitch, and one 64 pin .5 mm pitch . Both of these devices have a thermo pad on the back of them (pad area soldered to back of device)which i believe is affecting reflow. My initial reflow temp was 160 180 200 250 at 32cm/min. I tried dropping temp. of middle zones 5 degrees and upping peak 5 degrees. Tried this twice (changed twice), but still grainy joints. Any sugestions. We are using a kester water soluble paste.

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RDR

#28102

qfp reflow problems | 15 April, 2004

What are your temps at the locations that you are having trouble with. We cannot tell anything by the zone settings of an oven. We need max temp, how long above liquidous etc.. a lot of times this symptom is due to too long of a soak at too high of a temp or being in reflow for too long/hot. Also 5 degree change in zone temps usually accounts for almost 0 change in reflow temp. Most ovens only maintain +/- 5-10 deg. I would doubt that it is the thermal pad. We process 100s of 1000s of these on both .4mm and .5mm pitch from 64 pins to 216 with no issues. You need to put the ole thermo-couples on these joints and compare with paste recommendations.

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#28111

qfp reflow problems | 16 April, 2004

Hi Barry,

Your best course of action is to perform a thermal profile and determine exactly what the times/temps are in these locations. Try to match your profile to the recommendations of your solder paste. (Time between 130-160, Time over 183 and max slope) How does the rest of the board look? if it is limited to these locations and the thermal profile looks OK, it may be a component plating issue.

Good Luck

Chris

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#28120

qfp reflow problems | 17 April, 2004

Russ has a hint by saying "too long of a soak at too high of a temp or being in reflow for too long/hot".

I think the board should stay in the oven for 3-4 minutes, spending 60-90 seconds of which during reflow.

in order to stay in the oven for 3 minutes at 32 cm/min speed, the effective oven length inside has to be around a meter. From the recipe you gave, it seems to be a 4 zone oven, correct? Don't know the effective length though. Maybe the boards do stay too long in the oven.

If you don't have a PROFILER, have you ever tried sending the board without the QFPs and see how the solder joints look like? If dull again, definitely too hot. If nice and shiny as you will like, then maybe you can't heat the part enough.

Implementing the suggested profile is still the first step, I agree with Chris there.

Erhan

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barry

#28194

qfp reflow problems | 22 April, 2004

I placed thermo's at 2 of the problem devices. Soak (time 150 to 180) 100 sec. Time above 180 =80 secs with 208 deg. peak. The solder appeared to flow at these pads, but little to no adhesion to the joint.Total time in oven was 5 min. All other joints of other components were fine (plccs,chip comp.)my oven is a 4-zone, 65" length, Top bottom heaters. Hope this helps.

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RDR

#28197

qfp reflow problems | 22 April, 2004

Well so much for being too hot! Try kicking your max temp up to 220, I wouldn't extend the duration however. I would almost start to think that this QFP has a pb free finish. Let us know what happens

Russ

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barry

#28222

qfp reflow problems | 23 April, 2004

I have increased my peak to 220, and Flow is better, but it still not quite a good fillet all around. The solder tends to kinda "bunchup" at the toe of the lead. If part is Pb free, would this affect adhesion and or flow?

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RDR

#28223

qfp reflow problems | 23 April, 2004

The toes of leaded devices are problematic in fillet formation. A lot of the time they have the copper from the lead exposed so it becomes oxidized and won't readily take solder. Normally this is of no concern if the heel and side fillets are formed well. pb free finishes can and do have different appearance than those of tin lead. You will see the line of demarcation on these pb free leads. The solder joint should still be strong regardless of appearance. If these joints are still weak you may want to try a linear ramp to reflow to reduce the amount of soak. I noticed that your soak is at the long and high end of the spectrum. It is possible that you are driving off some portion of the flux prior to reflow. I have also found that if the lead finish is the TI nickel palladium it requires a max temp of about 225 and at least 60 sec. above 183 C to ensure that the palladium gets dissolved and you wet to the nickel.

Good luck Russ

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