|Ask the Experts|
February 24, 2021
Rework of Underfilled Array Packages
What is the best method for removing a SMT array package that has been underfilled and cured? Mechanically, thermally, chemically? A combination of methods? Is there is an industry approved method?
|Expert Panel Responses|
In previous years reworkable underfill systems tended to be very soft (even at room temperature),low Tg (glass transition) type materials used only for shock/drop reliability improvement. Recent material developments have led to higher Tg reworkable systems that provide the mechanical reinforcement as well as thermal cycling enhancement.
If the underfill material used was designed as a reworkable system, then the typical process would be to heat the component on a rework station some level above the solder liquidus (typically > 240C). This will melt the solder and soften the underfill (note: the underfill will not "melt" or completely decompose). While the device is at high temperature, twist the component.This can be done using a specially designed nozzle on a rework station or hand tools.
If there is sufficient space surrounding the component, the fillet may be scraped away (recommend using a non-metal tool for this to minimise solder mask damage) prior to component removal to make it easier. Once the component is removed, there is typically still underfill residue left behind.
Tacky flux / rework flux is typically applied and used to further soften the remaining residue and this mixture, along with any solder on the pads, can be removed using a vacuum tool. Following this up with an IPA wipe, the site is now ready for new solder, component, and underfill. This is a manually intensive process, that does require some level of skill to be proficient.
If you are using an underfill that has not been deigned to be reworkable. Then it is very unlikely you will be able to remove the component without damaging the underflying board. You can attempt the same procedure as above, but this will likely result in significant damage.
A few commercially available organic solvents can be used to dissolve and remove a cured underfill from under an area array package.Depending on the package dimensions and underfill type, a few applications of mildly heated Dynasolv 225 might work.
But before you start using any solvent to remove underfill, make sure the solvent will not damage neighboring components or materials within the package. Materials such as polyimides are highly susceptible. Of course, always wear lab PPE, follow laboratory safety guidelines and ensure proper disposal of used solvents.
CALCE, University of Maryland
Most underfilled array components will require a mixture of approaches both heat and mechanical energy will be required to remove the part. In order to facilitate the removal of the part is important to make sure that the solder is above its liquidus if it is not there is a tendency to remove the pads from the board.
Once the required temperature has been reach then the adhesion between the underfill and substrate must be broken this is usually done mechanically by either twisting the component ( if room allows) or prying the component up off the board.
After the component has been removed the site must be cleaned prior to replacing the part, this can be done with a variety ways but essentially the underfill is heated and scraped of the board. Finally it is desirable to give the site a wipe with an IPA soaked swab to remove any flux residues andl oose particles that could impact the performance of the underfill when the new part is assembled.
Senior Applications Chemist
Before attempting underfill rework, I recommend you contact the underfill supplier and find out if the material is truly "reworkable".
Underfill rework is always going to be challenging because the idea goes against the purpose of underfilling the component in the first place! Is it possible? Yes. So much will depend on the properties of the underfill you are dealing with.
In Finetech's experience, the process can be a combination of the methods you mentioned. We do have special machine mounted cutting tools to mechanically cut the fillet around the area package. This can also be done manually with great care not to damage the PCB. Follow this step with a suitable thermal profile based on the component, and with some mechanical assist (clamping nozzle in our case) you can remove the component.
Once the component has been removed, the process becomes more challenging.Removal of the residual underfill material will typically require a chemical. The material provider can recommend what to use to best remove the residual underfill. If successful, then you are ready to finish the site prep and solder a new component in place.
The answer to your question and rework approach depends on the type of underfill. We often find that the so-called "reworkable" underfills will indeed soften with heat; the device itself can be selectively removed from the PCB using a standard hot gas removal approach.
The underfill remaining on the PCB can then be carefully worked off with additional heat and agitation; our operators will use a soldering iron to break down what remains. It goes without saying that this must be done by a highly skilled operator to avoid damaging the solder mask, pads, circuits, etc. Once the underfill has been cleaned from the surface, you can proceed with stenciling paste on the prepared pads and new device placement. This has been a very successful approach in our experience when dealing with the "reworkable" underfills.
Non reworkable underfills will simply not soften or break down with heat compounding the challenge of rework. Mechanical removal of the device and remaining underfill via machining is a method that our company has employed numerous times with a high degree of success.
The assembly will be placed in a static bag with a window cut out of the bag to expose only the device to be removed. Care is taken to seal the edges of the window to the circuit board surface to prevent FOD from getting on unwanted areas of the assembly. The board is then fixtured flat on a grounded milling machine fitted with a carbide cutting tool. The operator then carefully machines the device off to expose the remaining underfill layer.
The underfill is then gradually machined off to within .003-.005" from the board surface. The remaining solder can then be wicked to expose the PCB pads prior to printing paste and device reinstallation. Once again, this very intricate operation requires a highly skilled and trained operator.
Circuit Technology Center
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