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Assembling Boards with BGAs on Both Sides



Assembling Boards with BGAs on Both Sides
What are the pros and cons of assembling circuit boards with BGA components on both sides? Should we first place BGAs on side one or side two? Does it make a difference? The Assembly Brothers, Jim Hall and Phil Zarrow, answer these questions.
Board Talk
Board Talk is presented by Phil Zarrow and Jim Hall of ITM Consulting.
Process Troubleshooting, Failure Analysis, Process Audits, Process Set-up
CEM Selection/Qualification, SMT Training/Seminars, Legal Disputes
Phil Zarrow
Phil Zarrow
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
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.

Transcript


Phil
Welcome to Board Talk with Phil Zarrow and Jim Hall of ITM Consulting. The Assembly Brothers.

Today we're coming to you from the Soldering Philosophy Room of ITM high atop here at Mt. Rialto where we're constantly pondering the question, what the flux.

Speaking of soldering I do believe we have a soldering design question today is that correct Jim?

Jim
I think it's actually a PCB layout thing, but the issues relate to soldering. This is from D.S. and it's simply, what are the pros and cons of processing BGAs single sided or double sided? First or second?

Well first off, let me say that this raises the wonderful question of how do we describe a double-sided assembly. First side, second side, top side, bottom side, side A, side B. Bottom, top.

I use the term bottom and top to describe its final configuration. The bottom side is the side that is soldered first and is therefore reflowed a second time when thetop side is reflowed. It doesn't matter what you call them as long as we all understand.

So that's the terminology that we will use today.

Phil
A lot of people look at the mass of the component and think they would definitely want to solder it on the second pass because of the weight of the component and the integrity of the joints when it is re-reflowed that second time.

We have found in our experience that's usually is not the real problem. There are other more intrinsic things, including exposure to asecond thermal extrusion and other reliability questions.

Jim
In general, people like to put only simple components on the bottom side to minimize all the problems that my brother alluded to. But it's generally felt that if you do a good job and the parts don't fall off it should be okay.

But there have just been some very good technical papers published that bring this into question for large complex BGAs on the bottom side, which see a second reflow cycle.

The potential problem is cracked solder joints, brittle fractures at the component solder ball interface during that second reflow cycle. The analysis seems to indicate that for larger packages, if too much warpage occurs before this bottom side second reflow it can crack that solder joint before it melts.

And because there's no flux present, those cracked surfaces will not resolder and come back together again, and you can have an intermittent or a very weak solder joint, or a total failure.

The conclusion of these authors was application specific and probably only works for certain large BGAs that warp a lot. It's much safer to put BGAs on the top side to eliminate the possibility of this failure mode occurring.

Phil
I think our mission should be to come up with acronyms fordescribing side one and side two, top side, bottom side. We can work on that. Anyway having said that, although the IPC tries to come up with a new acronym every time we say it, you've been listening to Board Talk. Whatever you do ...

Jim
Don't solder like my bother. 

Phil
And don't solder like my brother.

Comments

Why can't both BGAs be soldered at once? It seems that that would reduce the heat stress on the board.
Mary Ann Fay
Thermal mass, ball count, thermal sensitivity and complexity of a BGA are all factors in determining whether or not you should subject the BGA to a secondary reflow process.

As for top/bottom, we always relate to the sides as the exist within the CAD design itself. So, we do have a few boards that visually may appear to have all of the parts on the top, but they are actually on the bottom side of the board. Our manufacturing documentation would dictate that to be the bottom side, because that is what it is per design. It actually clears a lot of confusion, because anyone can reference the design and know the orientation intent.

We also have a few boards that run top side first and then bottoms side because of large inductors placed on the bottom side. These outweigh the risk (no pun intended) of a secondary reflow of the BGAs on the top side.

Lastly, when determining process issues, we always use IPC terminology of solder source and solder destination so that the top/bottom reference becomes irrelevant. It's about the process at that point, not the design anyhow.
Andrew Williams, PRIDE Industries
We use the terms "Conventional Component Side" and "Conventional Solder Side" or "Component Side" and "Solder Side" for short. This refers to the original Through Hole Double Side PCB Manufacturing.
Tony Upton, Schlumberger
We use "Primary Side" and "Secondary Side" on our drawings but in reality most people still seem to say "top" and "bottom" in casual conversation...
Pete Liebig, BAE Systems
Interesting issue - Which side is reflow soldered first and how we identify sides. I prefer using the side 'A', side 'B' to define sides. As far as which side is processed first, I prefer soldering the least complex side first but when both sides are equally populated with similar component types it really should not matter. Post assembly electrical test and inspection of the solder joints will be paramount and X-ray for the array packaged devices.
Vern Solberg, STC, USA
I suggest use the terms FIRST and SECOND side. It is always the SECOND side that is soldered upside down which is the true source of the issue. FIRST SIDE = Bottom SECOND SIDE = Top

This syntax completely defines the process. Leaving it to :Just something everyone understands is, as my Dad would say, "Cruisin for a Bruisin".
Bob Kondner, Index Designs, USA
BGA packages with back-to-back placement on double-sided design can be done and it is acceptable. However, the design is not preferred for considerations of rework, testing, and reliability. It is difficult to place thermocouples inside the center BGA solder joints to create rework profile. Also, there may not be space for test points under the BGAs. Also, inspections by x-ray images become impaired. Additionally, reliability of the product may be reduced due to rigidity of the double-sided area which prevents the board from flexing during thermal cycling. The stress would concentrate on the BGA solder joints and cause failures. Heavy BGA placed on the bottom side of the board may fall in the reflow during the top side reflow depending on pads design and how much solder paste is deposited on BGA pads. I had this issue with one specific PCB design. During the PCBA soldering 10% of heavy BGAs were falling into the oven.
Glayson Figueiredo, Philips Medical Systems, Brazil

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