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Micro solder balling

Antonio A. Medina

#2643

Micro solder balling | 26 October, 2000

Ok guys, I need help. Here's the deal. We're getting micro solder balls under discreets on the bottomside of the board. Here's my set up...

MPM screener w/vacuum plate 6 mil metal screen with 90% circular (rounded) apertures 7 mil (measured) paste Paste = Kester CENTCH 0445 R256 (63%/37%) No Clean Visc. Ps. 1100-1800 Humidity is normal (whatever that actually is) Part placement with a CP-643E. Reflow with an Electrovert OmniFlow 7 with 7 zones: 130, 160, 160, 170, 190, 240, 240 for both top and bottom convection, moving at 26 in/min, using Air environment.

We're building to class 2 requirements, the board is 8"X11" with 682 discreets being put on the bottomside. What happens is that tiny solder balls are all over the board right next to the component body. IPC has "imbedded" in the paste standard. So, theoretically, were OK, but I just KNOW the customer is going to have fits. Going to the stencil with round aperatures helped but didn't fix it. I ran a bare board thru to see if the masking was the problem; not one solder ball. The nozzle placement of the CP is also good, it's not coming down too far. What's everyones take on this?

Thanks in advance, Antonio

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NLykus

#2644

Re: Micro solder balling | 26 October, 2000

Antonio,

I would need to have more profile information i.e. preheat temp (hold time) as well as soaking zone (time). What is the peak temperature and how long is it held there.

Also, one more thing to check, because I have seen it in the past, is what type powder is being used to make the paste? Type 3 or 4. This can make a dramatic difference.

If you have profile information, please advise. Nance

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#2645

Re: Micro solder balling | 27 October, 2000

Antonio: A lot has been said on this topic before. You might try the archives with "solder balling" and than check your process step by step ( you will find enough ) to find the cause of your problem. Any other advice would be repeating and repeating the already said again and again.

Good luck

Wolfgang

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Dr. Kantesh Doss

#2646

Re: Micro solder balling | 31 October, 2000

Hi Antonio: You clearly have printed excessive amount of solder paste on the pad. You can reduce the amount of paste by one or two methods: 1. Switchover to 5 mil thick stencils. 2. Change the design of your stencil openings (at the discrete component locations)to home plate design. Contact me at my e-mail if you need more info on a special home plate that has worked well for me.

Also, check your reflow profile. The temperature in the reflow zone appears to be on the high side. Try the following process conditions which should work for most of the no clean solder pastes: 1. Peak Temperature: 220-225�C 2. Time above 183�C : 50-60 seconds 3. Soak Time in the range 130-160�C: 120-140 seconds 4. Max. Heating Slope: 3-4�C/sec.

Another point I want to make is that try minimizing the placement force so that there won't be any "squeezing out" of the solder paste.

Finally, if you are willing to go with a drastic measure, try some other no clean solder paste from another vendor.

Hope the above helps.

Kantesh Doss New Process Introduction Specialist Nokia Mobile Phones

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