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Solder Balls-No Clean Paste

Views: 5519

#62451

Solder Balls-No Clean Paste | 17 August, 2010

I have a question that has been discussed many times, but is always good to get some new insight. We have lots of issues with our no clean process. We use a Kester EP256 solder paste. I would just like to get some general tips on how to get rid of mid chip balls. Please offer any suggestions or tips that you have.

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#62452

Solder Balls-No Clean Paste | 17 August, 2010

The best way that I have found is to use homeplating on my stencils.

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#62455

Solder Balls-No Clean Paste | 17 August, 2010

That reminds me...We already have home plate apertures on our stencil. Try number 2...I tried an overall reduction based on the copper layer of 10% and used oval apertures. I hope that you guys can suggest something that I am over looking. I know that once you get frustrated you start to over look things.

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#62456

Solder Balls-No Clean Paste | 17 August, 2010

Since you have home plated the apts and done solder reduction, I would take a look at your soldering profile. We have run into this before when running no-clean and found that reducing the initial ramp rate to the flux activation was a help in the reduction of both mid chip balls and the random scattering as well(of course).

Sounds like you have started in the right direction...

'hege

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#62457

Solder Balls-No Clean Paste | 17 August, 2010

Comments are: * Too much paste ... reduce it more. * Do not let a traces run between small discrete pads or the heat of reflow will pull a ball to the trace, we can repeat this. * Change the placement height on chip parts to reduce solder spread / mushroom effect. * Try another paste * Remove the solder mask between the pads on small chip components (???) * Fix preheat and soak slope and dwell of reflow recipe

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#62459

Solder Balls-No Clean Paste | 17 August, 2010

Are you controlling humidity in your plant? I'm not familiar with that particular paste, but some pastes are sensitive to a humid environment. We also had one paste supplier send us a batch that had excessive solder balls if the boards were printed then left sitting for about an hour before placement and reflow. The supplier had made a process change that caused it to be time sensitive.

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#62467

Solder Balls-No Clean Paste | 18 August, 2010

> Are you controlling humidity in your plant? I'm > not familiar with that particular paste, but some > pastes are sensitive to a humid environment. We > also had one paste supplier send us a batch that > had excessive solder balls if the boards were > printed then left sitting for about an hour > before placement and reflow. The supplier had > made a process change that caused it to be time > sensitive.

No the climate is not very controlled. We are trying to adjust our profile a little to see if that helps. So far it hasn't been very effective.

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#62468

Solder Balls-No Clean Paste | 18 August, 2010

So with your mounting frustration in your inability to zero-in on the factors that drive your solder balling, are you thinking that maybe you need to bite the bullet and design a experimental study to help isolate the key drivers to your solder balling and interactions between drivers?

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#62475

Solder Balls-No Clean Paste | 19 August, 2010

I have seen that moisture in pcbs will cause mid chip solder balls. To discount this, take one pcb from a batch that is giving you solder balls and pass it through you reflow oven with no paste or components on (bare pcb)using your normal reflow profile for that pcb....this is a quick way to dry out the pcb to the level where hopefully most of the moisture will have been evaporated from the pcb. The process this pcb as normal and see if you have solder balls. I had one paste user who never got solder balls, but on one pcb they were on every chip. We checked and double checked everything like paste volume / placement pressure and reflow profile and then finally 'dried' the pcb as above and the solder balls vanished. If I hadn't have seen it myself I would not have believed it....the moisture in the pcb wasn't causing any other defects, just mid chip solder balls.

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#62480

Solder Balls-No Clean Paste | 19 August, 2010

Thanks everyone. I have thought about the moisture in the paste a lot, but never really gave the raw card moisture any consideration. The next time we run an assembly with the issue I will try baking some of the cards once the issue shows up to see if it goes away.

Lots of great help so far. I can't thank you enough. This is the number one issue right now that I can't get to go away and stay away.

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kpm

#62481

Solder Balls-No Clean Paste | 19 August, 2010

Are you reusing your paste or putting down new every time? I've seen solder balls before when operators try to use the paste past the recomended stencil life. I've also seen it happen with a bad batch of solder paste from a vendor.

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aj

#62487

Solder Balls-No Clean Paste | 20 August, 2010

Hi,

I would recommend you trial an alternative supplier if you have the option.

I did a lot of trials on the issue and dont have any problems.

Contact me off line and I will share details

aj...

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#62488

Solder Balls-No Clean Paste | 20 August, 2010

kpm,

We were afraid of that, so we have tried several variations with paste usage. New paste, transfered paste from another line, and a new tube from another date code. I think the problem is profile, board design or humidity. I just have to starting removing some of those options now. Thanks for the tips.

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#62489

Solder Balls-No Clean Paste | 20 August, 2010

Hello, I have the same problem ..., please take a print screen with the profile to see how is made it. Thank !

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#62492

Solder Balls-No Clean Paste/ Article on Stencil Design Guideline | 20 August, 2010

Article to assist with Stencil Design Guidelines http://shanelo.co.za/technical_paper_about_stencil.htm

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#62498

Solder Balls-No Clean Paste | 21 August, 2010

I have noticed that the % metal content in the paste can also be directly correlated to solder balls (less metal, more balls). However, I also found differences between pastes from different manufacturers and the number of solder balls (identical boards and profiles). Because I do not layout all of the boards I run, I needed a paste that is forgiving of layout problems.

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