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May 8, 2017

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?


Experts Comments

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.
Dr. Brian Toleno
Application Engineering
Henkel Electronics
Dr. Brian Toleno is the Application Engineering Team leader for Henkel Technologies. He is responsible for the technical service and application engineering for Henkel's electronics assembly materials, including solder paste, underfills, PCB protection materials, and underfills.
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.
Bhanu Sood
Laboratory Director
CALCE, University of Maryland
Bhanu Sood is the Laboratory Director at the Center for Advanced Life Cycle Engineering (CALCE) and actively assists companies and organizations in all aspects of electronics reliability. Sood's key focus area is in design reviews, custom tests, and failure analysis services. He has authored several articles on board and component level reliability and unique failure mechanisms in electronics.
Most uderfilled 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 and loose particles that could impact the performance of the underfill when the new part is assembled.
Neil Poole
Senior Applications Chemist
Henkel Electronics
Dr. Poole is a Senior Applications Chemist in Henkel Technologies, electronics assembly materials application engineering group. He is responsible for all of Henkel's assembly products including soldering products, underfills, PCB protection materials, and thermally conductive adhesives.
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.
Neil O'Brien
Sales Director
Neil O'Brien has worked in the field of electronic manufacturing equipment for over fifteen years and is currently Sales Director for Finetech, a manufacturer of precision rework systems and die bonders.
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