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November 1, 2018

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?

E.C.

Expert Panel Responses

In previous yearsreworkable underfill systems tended to be very soft (even at room temperature),low Tg (glass transition) type materials used only for shock/drop reliabilityimprovement. Recent material developments have led to higher Tgreworkable systems that provide the mechanical reinforcement as well as thermalcycling enhancement.

If the underfillmaterial used was designed as a reworkable system, then the typical processwould be to heat the component on a rework station some level above the solderliquidus (typically > 240C). This will melt the solder and soften theunderfill (note: the underfill will not "melt" or completelydecompose). While the device is at high temperature, twist the component.This can be done using a specially designed nozzle on a rework station orhand tools.

If there is sufficient space surrounding the component, thefillet may be scraped away (recommend using a non-metal tool for this tominimise solder mask damage) prior to component removal to make iteasier. Once the component is removed, there is typically still underfillresidue left behind. Tacky flux / rework flux is typically applied andused to further soften the remaining residue and this mixture, along with anysolder on the pads, can be removed using a vacuum tool. Following this upwith an IPA wipe, the site is now ready for new solder, component, andunderfill. This is a manually intensive process, that does require somelevel of skill to be proficient.

If you are using anunderfill that has not been deigned to be reworkable. Then it is veryunlikely you will be able to remove the component without damaging theunderflying board. You can attempt the same procedure as above, but this willlikely result in significant damage.


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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 usedto dissolve and remove a cured underfill from under an area array package.Depending on the package dimensions and underfill type, a few applications ofmildly heated Dynasolv 225 might work. But before you start using any solvent toremove underfill, make sure the solvent will not damage neighboring componentsor materials within the package. Materials such as polyimides are highlysusceptible. Of course, always wear lab PPE, follow laboratory safety guidelines andensure proper disposal of used solvents.

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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 arraycomponents will require a mixture of approaches both heat and mechanical energywill be required to remove the part. In order to facilitate the removal of thepart is important to make sure that the solder is above its liquidus if it isnot there is a tendency to remove the pads from the board. Once the requiredtemperature has been reach then the adhesion between the underfill andsubstrate must be broken this is usually done mechanically by either twistingthe component ( if room allows) or prying the component up off the board. After the component has been removed the site must be cleaned prior toreplacing the part, this can be done with a variety ways but essentiallythe underfill is heated and scraped of the board. Finally it is desirable togive the site a wipe with an IPA soaked swab to remove any flux residues andloose particles that could impact the performance of the underfill when the newpart is assembled.

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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 theunderfill supplier and find out if the material is truly "reworkable".

Underfill rework is always going to be challenging because theidea goes against the purpose of underfilling the component in the firstplace! Is it possible? Yes. So much will depend on theproperties of the underfill you are dealing with.

In Finetech's experience, the process can be a combination ofthe methods you mentioned. We do have special machine mounted cuttingtools to mechanically cut the fillet around the area package. This canalso be done manually with great care not to damage the PCB. Follow thisstep with a suitable thermal profile based on the component, and with somemechanical assist (clamping nozzle in our case) you can remove the component.

Oncethe component has been removed, the process becomes more challenging.Removal of the residual underfill material will typically require achemical. The material provider can recommend what to use to best removethe residual underfill. If successful, then you are ready to finish thesite prep and solder a new component in place.

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Neil O'Brien
Sales Director
Finetech
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.

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.

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Bob LePage
Sales Engineer
Circuit Technology Center
Mr. LePage has been a key member of the team at Circuit Technology Center since 1996. He has vast expertise, experience and understanding of complex circuit board rework, repair and modification operations. He is one of the most knowledgeable experts in this area across the globe.