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Poor solder joints on QFP 100's

#8217

Poor solder joints on QFP 100's | 11 December, 1999

We have a pretty challenging board (2-up panel, 15"x20", 48 20mil pitch QFP 100's and 2 25mil QFP128) that gives us LOTS of lead bridging on the 20 mil parts. There is typically very poor wetting on these parts also, and some joints just flat aren't, IOW, they are butted, but not connected. All other components, including the passives and the big QFP) exhibit good joints.

From your experience (and I understand that there will be a variety of answers here), is this most likely a reflow profile problem, a paste problem, an oxidation problem (component or pad), or other problem? Thanks, guys.

Steve

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#8218

Re: Poor solder joints on QFP 100's | 13 December, 1999

Hi Steve, 20 mil pitch shouldn�t be that problem nowadays. In your case bridging and poor wetting seem to go hand in hand. For the poor wetting try to eliminate it by using different paste (higher activated) and work on your profile. Coplanarity also seems to be a problem in your case. Bridging also occurs due to misaligned printing and to much paste deposits. Check your stencil, a lasercut 6mil stencil with apertures reduced about 10-15% works great for us with 20mil parts. There was a thread not long ago on this topic. How is your pad design ? IPC offers an online calculator, I have experienced that poor pad design always makes thing for the assembly ugly.

Good luck

Wolfgang

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Chris May

#8219

Re: Poor solder joints on QFP 100's | 13 December, 1999

As Wolfang says about slightly bent legs, handling is of the essence.

Are these devices decanted into another tray for your machine. It may be worthwhile doing a "movement" chart for these components. Do not explain why or when you are doing it because you will get a false "someones watching me" reading. It could be a simple (but expensive) handling problem. If it is do not make a fuss. Just educate. Store people are human to.

OR

A) Have you profiled this board correctly (yourself that is) B) Have you got some "shadowed" areas due to compact density C) Is your board finish / paste compatible (Immersion Gold) D) Has anyone checked footprint / Pad size ref IPC 782

Regards,

Chris.

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#8220

Re: Poor solder joints on QFP 100's | 13 December, 1999

Thanks, gentlemen. All points well taken, and some hit pretty close to home. Our pick-and-place programmer assures me that coplanarity isn't the issue because the machine won't (or can't) place parts beyond an acceptable criteria. It's an Amistar 5530L, and apparently one of it's sensing functions screens for coplanarity. I know nothing beyond that, but that will come with time. If you know he's wrong, please let me know.

The stencil is a brand new design for us, 5 mil, 1:1 apertures/pads, traps, and is much better than our previous stencil w/respect to release. I don't think the stencil is the problem because, although we've changed stencils and get much better performance print wise, the results out of reflow are the same.

I'm more inclined to think it's the profile, especially since I've spent some time with it today. I think the oven was profiled with a bare board, w/taped down tc's. I ran soldered-to-lead-bottom profiles today and the max. temp reached on 3 out of the 4 locations didn't pass 210C, and one was below 200C. After I played with the heat ranges I got it to look much better but I'm concerned about to much heat in the reflow chamber. Is going from a setting of 200 in zone 5 to 270 in zone 6 (reflow) to much of a jump? That's what I need to get to a decent max. temp. without exceeding the reflow temp for too long. Now much can I shock these components by doing that? I haven't looked into the manufacturers specs. for max. temp. but I guess that's a good place to start...

So much to do, so little time.....and knowledge.....but that's probably becoming painfully obvious to you. Thanks for any and all assistance, guys.

Steve

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Curtis T.

#8221

Re: Poor solder joints on QFP 100's | 13 December, 1999

Steve, I have done countless board profiling and oven reflow profiles. Here are the critical factors on you need to address. What is your board surface finish, if it is something like an OSP it will need a good amount of an active (aqueous) flux to eat through the coating and let you solder to the pads. Gold finishes like it a little hotter and a little longer. Each surface finish has its own preferences. Next and most importantly what is your paste manufactures recommended profile for the paste. I would go with one of the big names (I have used them all Kester, Alpha, Indium) because each of them have pastes that perform excellent on the board finishes of today. In creating your reflow profile remember that if you give the paste too much heat up front it will burn the flux off early. If you don't have a long enough soak (activation) time the flux will not have time to do its magic. Typically the ramp should not be more than 2'C/sec. And lastly if your components have an unusual lead finish it can cause real problems. For example if you have a palladium finish that is rather thick you have to allow longer for it to melt off the lead and migrate into the joint or you will have very poor wetting. I do not remember if you said were the solder was ending up. Was the solder all stuck to the lead, or was it all on the pad and none on the lead, this will tell you right away were to concentrate your efforts. Hope this helps Curtis T.

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Chris May

#8222

Re: Poor solder joints on QFP 100's | 14 December, 1999

Steve,

Two good reference books to have are

1) SMT Soldering Handbook by Rudlof Strauss (ISBN 0-7506-3589-4) 2) Soldering In Electronics by R.J. Klein Wassink( ISBN 0-901150-24-x)

Regards,

Chris

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Mike Naddra

#8223

Re: Poor solder joints on QFP 100's | 16 December, 1999

Steve , I use a general set of guidlines when developing reflow profiles: Ramp rate to 140 deg C - 50-60 seconds Time at preheat (140-160 deg C) - 90-120 sec Time above liquidous (183 deg C) - 45-90 sec Time at peak (215-225 deg C) - 9-12 sec peak temp 225deg C Max slope to pre heat - 2deg C / sec Total elapsed time to peak 5-6 min

You ask if jumping from 200 deg C to 270 deg C would shock the component, you need to look at the temperature slope. How many zones does your oven have? Some things to consider: You probably dont want to exceed 3 deg C /Sec - this could possibly shock the components. You do not want to be in preheat to short or to long - to short the flux does not have enough time to work and to long we start to burn the flux off and that affects wettability at reflow. You want to ensure you are above liquidious long enough - this is where the intermetallic bond is formed,solder wetting and grain growth occurs. You want to pay attention to elapsed time to peak. What belt speed are you set at? What zone are you reflowing in?

Mike

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