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BGA Voids

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#53613

BGA Voids | 11 February, 2008

What causes BGA voids over 25%? Have recommended profile for solder paste, hit 237C for 5 seconds and still get a void on 2% of specific BGA in various locations. BGA is FG676, 1mm pitch, on ImAg using lead free paste OM338 on a 6 layer FR4 PCB.

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#53615

BGA Voids | 11 February, 2008

While you're waiting for others to reply, search the fine SMTnet Archives to find things like http://www.smtnet.com/forums/Index.cfm?CFApp=1&Message_ID=41292

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aj

#53616

BGA Voids | 12 February, 2008

Hi,

Have you checked out the BGA datasheet and see what profile is recommended for the BGA ?

I use OM338 and there are dozens of recommended profiles for the Paste but you have to consider the components aswell.

You state your Peak temp which I would say is a bit on the low side ( I would hit 240-245 for this type of BGA).What is your TAL?

Also, note that some voiding is not necessarily a bad thing.

aj...

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#53621

BGA Voids | 12 February, 2008

I must ask the silly question- are you sure you deposited the correct amount of paste? Depending on aperature design-the voiding could come from a stencil that has its holes partially built up with old paste.

If paste volume is guaranteed, then what makes paste "void".

Outgassing comes to mind - are you seeing lots of little solder balls under the BGA, indicating the solder paste blew like a roman candle prior to liquidous.

Is your paste/flux ratio in control? Too much flux can mean greater outgassing.

Are your fab.'s dry?

Lots more to consider, gotta take it all one at a time. Eliminate the variables until you find the common cause(s).

Are you able to run a profiler with thermal couples through the PCB and into the paste/ chip ball interconnect?

Watch the ramp rate- are you exceeding 2 degree C per second?

Ok- so I exceeded 2 cents worth- with inflation I figure I have 1.5 cents credit to use.

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#53622

BGA Voids | 12 February, 2008

Thanks for the spare change. Questions are good and I did miss one item. Process has 100% solder paste inspection with a 3D inspection machine that has very few false calls or accepts. Ramp to peak is less than 1.5C/sec. Profile looks good, solder ball has thermal couple inserted through bottom of PCA. PCB's are dry, consume a sealed package of PCB's in 30 minutes to 4 hours after opening. Will pry off parts to check for solder balls around void. Solder paste has varied a little in viscosity but voids do not track lot changes. No umicro via's in pad.

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#53625

BGA Voids | 13 February, 2008

The causes of voids are extremely paste dependent. I did a literature search a couple years ago and the papers all disagreed with each other - one would say turn knob "A" to the right and the next would tell you to turn it to the left. I did my own experiments with multiple paste suppliers and found the same thing. Factors that effect BGA voiding with one paste would have no effect or the opposite effect with another paste. You'll probably have to do your own experiments since even the profile recommended by your paste supplier probably wasn't developed using all the same factors (such as board surface finish) that your design contains.

Another issue is measurement. I've seen some pretty high end x-rays fail a Gage R&R for void measurement. Hate to throw out parts or monkey with my process based on an unstable measurement system.

At the end of the day, voids don't effect reliability anyway. I work in the high reliability market and I've done enough testing to convince myself of it. Everything published in the industry (despite IPC-7095) comes to the same conclusion. Unfortunately, IPC published limits for BGA voids so now I have to waste effort with our customers.

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#53626

BGA Voids | 13 February, 2008

Larry, from my days of working for a Telecom. company, Bellcore standards (now Telcordia) was our BGA Void guideline. There was some verbiage in the guideline that accounted for allowable VOID VOLUME ...keep in mind, VOID AREA and volume are 2 different things.

The 2D x-ray that we used measure void AREA (not volume). That being said, we took the area measurement, and coverted it to volume - example 13.5% area void translates to 5% volume void (it's all from your high school gee-i'm-a-tree class). The QE's in your company will love this. See attachment for details.

(THANKS SMTNet for adding this feature :-) )

Attachments:

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