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? Board Talk
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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.
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.
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.
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?
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.
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.
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 ...
Don't solder like my brother.
Don't solder like my brother either.
I would also add not only you have the coplanarity issue you also have no control of the amount of solder on each pad as HASL will not be consistent. What type of solder does the board house use for the HASL? You will then add paste and create varied volume of an unknown alloy cocktail of sorts. I Would suggest ENIG.
Darren Pieczynski, Burton Industries
The smaller the ball diameter of the BGA, the more standard HASL can affect finished solder joint uniformity between the BGA and the board pads. HASL is not a uniform finish, it can vary wildly from pad to pad and from one section of the PWB to the another due to variations in the air knives leveling the solder and other factors. Some fabricators/processes are better than others also. My general rule of thumb is that for BGA ball sizes greater than about .030" diameter and edge-to-edge ball spacing of about .010" or greater, HASL finish is generally just fine. but for ball diameters or spacing less than that I would not use HASL.
You are speaking of a .024" diameter ball with unknown edge-to-edge spacing. The ball diameter is now getting so small that variations in the molten HASL volume relative to the small ball diameter will cause greater variation overall, as well as less forgiving alignment during reflow. This leads to possibly tilted BGAs and possible misalignment due to greater pulling force differences from surface tension during the liquidus stage.
Richard Stadem, General Dynamic Mission Systems
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?
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.