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Pb Free Voids

Views: 2486

#47626

Pb Free Voids | 13 February, 2007

Fellas:

I'm having a tough time with voiding with my lead free water soluble paste under my large BGAs. I know some is almost inevitable, but this is more than I'd like. This is with a slope of .9, soak of 60 seconds, above 217 for 85 seconds, peaking at 243C. (This profile was developed with someone from the paste manufacturer) Any thoughts?

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#47630

Pb Free Voids | 13 February, 2007

There are solder pastes that seem to give you less voiding. Currently we are using Kester ,water soluble paste that gives us pretty good results on BGA's up to 0.5mm pitch. Stencil design was a challenge but we are getting consistent results . Here is a link to the data sheet. http://www.kester.com/Data%20Sheets/Solder%20Pastes/Water%20Soluble/R520A%20Global%20(07Sep05).pdf

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dave

#47631

Pb Free Voids | 13 February, 2007

Rob,

What do you mean by stencil design was a challenge?

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Wayne

#47632

Pb Free Voids | 14 February, 2007

The middle of the BGA has to make sure it has 235 deg C (if you are using SAC paste), otherwise it will not form good joint.

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CK the Flip

#47634

Pb Free Voids | 14 February, 2007

I agree with Rob that voids are often times paste-dependent, and it's all about the type of flux that's used. The cause of the voids, as you all know, is outgassing of flux which has nowhere to go with BGA's. Some paste mfgrs. will recommend going to an extended soak to dry out the volatiles. Be careful there, though, 'cause you don't want to dry out all your activator before spike.

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#47635

Pb Free Voids | 14 February, 2007

The challenge regarding the stencil was mostly due to the actual board layout and nothing having to do with the voids in the solder. It was my first encounter with 0.5 mm pitch on BGA and dealing with a LF solder paste with higher viscosity than tha leaded version previously used on the same design. This project was an inheritance, lol.

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CK the Flip

#47636

Pb Free Voids | 14 February, 2007

Rob, just curious how did you overcome this through stencil design? Did you play around with foil thickness and aperture geometry?

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#47637

Pb Free Voids | 14 February, 2007

This was a rather intersting little project to work on due to the fact that I had such a variety of components placed in SMT ( your regular 0402 passives, ,BGA's Qfp's ,TH and SMT connectors ,plus the pesky micro BGA) I ended up with a 5/3 mil electroformed stencil with squared appertures for my various BGA's and some pre-forms added for the extra volume needed for the TH parts. A nice flat Immersion Ag helped too. It turned out to be a very successful trial( good thing too, as I'm doing contract work while waiting for bigger/better things).

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CK the Flip

#47642

Pb Free Voids | 14 February, 2007

5/3 mil as in... 5 mils stepped down to 3 mils?

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SRM

#47643

Pb Free Voids | 14 February, 2007

50/50

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#47644

Pb Free Voids | 14 February, 2007

Thanks for the input

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#47655

Pb Free Voids | 15 February, 2007

Thanks for the help guys. I do appreciate it!

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