Best Method for Repairing Underfill Arrays?

Phil
And welcome to Board Talk with Jim Hall and Phil Zarrow, the Assembly Brothers. What interesting, provocative question do we have today Jim?

Jim
You see topic about rework. It comes from E.C. What is the best method for removing an SMT array package that has been under-filled and cured. Mechanically, thermally, chemically, a combination of these methods. Is there an industry approved method? 

Phil
When all else fails go look for an IPC spec to tell you how to do it. One thing you could do is trash the board, but that's probably not an option here. That would be the easiest. 

Jim
I think the reality is a lot of boards get trashed because this is not an easy process.

Phil
The answer to the question really depends on the nature of the under-fill itself.  Whether it's thermoset, thermoplastic. You heat it up being thermal plastic if it gives ... but my thinking is you better have a chisel handy.

Jim
I find that most of my repair technicians say that, they use a twisting motion, gripping the component in some sort of holder and rotating as well as pulling rather than trying to pride up from the edge.

But it's not an easy process, we would be kidding you. Certainly any process that involves some mechanical energy and as my brother Phil says depending on the material some heating may help to some extent. With some materials it may not, so you really should talk to the supplier of the under-fill material if that knowledge is available.

Phil
Therein lies the real solution.

Jim
And the reality is a lot of boards are going to be torn up and probably damaged beyond repair. Such is the nature of the beast.

Phil
You might have a little bit of reprieve if you have only done the corner bonding of the under-fill which people do on flip-chip. That's one of the limitations of the flip-chip or under-filling anything. 

Jim
Oh finally, I know of no chemical methods for aiding in the removal of under-filled parts.

Jim
Whatever method you use around removing your under-fill array packages, when you go to put them back on, don't solder like my brother.  

Phil
And sure don't solder them like my brother.

The approach our company will take to reworking underfilled BGA's depends on the type of underfill material used. Some underfills are "reworkable" and will soften enough with heat to allow for safe device removal via our hot gas rework equipment. Any remaining underfill left at the BGA site after device removal is carefully worked off with a soldering iron. A skilled operator with the proper touch is required to avoid damaging soldermask, pads and the base material. After removal, the site can be selectively screen printed with solder paste and new device installed.

We utilize a BGA device mill-off process when dealing with "non-reworkable" underfills, which do not soften at all when heated. A high level of operator skill and precision milling equipment is required, however we have successfully reworked thousands of very valuable circuit boards following this method. A description of this process, step by step procedure and video can be found on our rework and repair guidebook, https://www.circuitrework.com/guides/9-5-1.html

Speaking of Neanderthal approaches, we have one client who owns high end CNC machining equipment. In a few cases they have machined the part off the board, grinding away both the chip and underfill down to about 2-3 mils height above the board. Then they sent the board to us for replacement, which was a snap, as all the pads had nice pockets to aid with alignment, and virtually no risk of bridging. Obviously this approach is not for the faint of heart, and requires access to high end machining. And it requires some masking and cleaning to remove the dust, which is potentially conductive. Compressed air would be sufficient, I think.

For most applications, though, I agree with the assessments above, that some twisting and prying are required, which can be a little tricky during a reflow profile. Bottom line is, when reworking underfilled components, don't guarantee yields.

You can avoid cured rework and manual processes by eliminating underfill and using a dry pick and place alternative. Alltemated Inc. manufactures Place-N-Bond Underfilm that precisely places the right amount of adhesive where you need it. It is 100% reworkable and requires no secondary processes or curing. Reflow at 135 degrees, the film liquifies again and remove the component. Clean the board with a solvent, re-apply paste and the Underfilm strips then reflow.

I remove BGA components from PCBs on a daily basis with underfill for the most part I am successful every now and then we get a device that really is just a pain, 2 methods I use 1 is with heat I do it using a heat wand and a pen knife to pry up on the component to "pop" it off this is most efficient way I have found but not all components on the PCB with stand the heat well, in the more sensitive situations I use a mill to mill the component off of the PCB being careful not to damage the solder pads or PCB this is only effective if you do not need the component you are removing but it does help reducing the reflow cycles during rework. I have been looking and researching trying to find a chemical that could soften the underfill to make removal and clean up easier. Was just curious if anyone knows of one.

We use under-fill under large BGAs. According to the under-fill material supplier, heating the component to just above T-liquidis of the solder, then twist/rotate and remove the component. Obviously the pads need to be cleaned afterward prior to installation of a replacement. We found that heating the opposite side of the board to around 180C helps to minimize the heat stress on the board. The technique works fairly well - we use a BGA rework machine to create the heat and profile.