Electronics Assembly Knowledge, Vision & Wisdom
BGA Components and Coplanarity
BGA Components and Coplanarity
We have a PCB with a HASL surface finish, the assembly includes a 256 ball BGA. The component balls are .6 mm pitch. Is coplanarity a concern? Jim Hall and Phil Zarrow, The Assembly Brothers, answer this question.
Board Talk
Board Talk is presented by ITM Consulting

Phil Zarrow
Phil Zarrow, ITM Consulting
With over 35 years experience in PCB assembly, Phil is one of the leading experts in SMT process failure analysis. He has vast experience in SMT equipment, materials and processes.


Jim Hall
Jim Hall, ITM Consulting
A Lean Six-Sigma Master Blackbelt, Jim has a wealth of knowledge in soldering, thermal technology, equipment and process basics. He is a pioneer in the science of reflow.

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Transcript
Phil
And welcome to Board Talk with Phil Zarrow and Jim Hall, of ITM Consulting. Otherwise known as The Assembly Brothers.

Today we're coming to you from the ITM Consulting Broom Closet. We're high atop Mt. Rialto and we're here to talk about electronic assembly. Materials, equipment, components, practices procedures, goof ups, and other things.

Today's question I believe has to do with surface finish.

Jim
It comes from T.Y. We are an EMS contractor. We have a customer that has requested a PCB with a HASL that's hot air surface leveled, surface finish.

The assembly includes a 256 ball BGA, the component balls are .6 mm.  Should we be concerned with coplanarity?

Are there any other concerns?

Phil
Yes! We do not like HASL for purposes, because of the coplanarity issue right off the bat. You have to understand that hot air solder leveling and hot air surface leveling or hot air leveling, whatever you want to call it, HASL, it originates back to the through hole days.

With through hole we didn't really care about the coplanarity or topography of the surface of the board. Surface mount, we care about it a lot.

So it's very important that we have a very coplanar surface. We start taking about things like bottom terminated components and ball grid arrays and area arrays, it becomes extremely paramount that we have a nice level surface.

Because of the nature of it, the fact that you're using hot air as essentially a squeegee on the surface of the board it's anything but coplanar. There's a lot of variation in it. I don't care if you're talking about leaded or lead-free HASL. It's still an issue.

It's not a great methodology to use for surface mount. There's even a question about what it's doing to the integrity of the surface of the circuit board. 

Jim
Another general issue that's raised is the ability to screen print reliably on surfaces that are not flat. Since most people, particularly with larger pitches as this indicated here with the .6 mm ball probably indicating a one mm or40 mil pitch part, that would allow you to use area reduction on your stencil apertures to assure good gasketing.

Gasketing between a rectangular aperture and a domed surface on the pad is going to be problematic in any case, so that's another issue independent of components.

If you don't get good gasketing, solder paste can squeeze out under the pads and give you bridges and solder balls.

Phil
My question is why is this informed customer requesting the HASL surface finish? We think HASL's a bad idea anytime surface mount is involved. There's your answer.

You've been listening to Board Talk. What ever you do, what ever you're soldering, what ever the finish ...

Jim
Don't solder like my brother. 

Phil
Don't solder like my brother either. 

Comments
Usually BGA coplanarity as per manufacturer will be 0.1 mm, if it is beyond 0.1, part is is out of spec. To manage 0.1 coplan of BGA, try to use stencil with over printing square rounded corner till solder mask so that this will manage 100% ball wetting and head on pillow issue.
Parasu, Sanmina, India
The usage of any non-programmable HASL for replacing BGAs, and mostly BGAs having these balls and pitches, is the wrong method. This is not reliable and repeatable.

This lead in 80% to:
- Overheating the new BGA and the neighboring components, as well
- Cold soldering joints of the new BGA
- Damaging the new BGA (reduction of the life time due to overheating)
- Higher reclamation as the device in question may fail after some few months by the user (leading very low customer satisfaction

Here some background question from my technical know-how and a trainer in lead and lead-free soldering:

- How are you aligning the new BGA (manual or with a vision system)?
- How would you know that all solder balls under the BGA have been really melted and completely soldered to the PCB?
- What is the temperature on top of the new BGA?
- Which kind of nozzle are you using?
- What happened to the neighboring components by the visual inspection?
- Are you able to x-ray the new reworked BGA?

Note:
I would recommend you to use adequate BGA soldering equipment with optimum developed soldering profiles by using the corresponding soldering nozzle with an adequate airflow for this rework, for meeting the customer satisfaction and keeping your reputation.
Anani Atiye, Open fornue opportunity, Germany
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