Underfill for Area Array Packages

What Underfill Does

Underfill is a filled epoxy that is dispensed along the edge of a mounted package and flows by capillary action into the gap between the package and the board, filling the space around every solder joint. Once cured, it mechanically couples the package to the laminate and shares the stress that thermal expansion would otherwise put entirely on the solder joints.

The effect on reliability is large. A ball grid array without underfill concentrates all the thermal expansion mismatch into the solder balls, particularly the corner joints, which is where fatigue cracks start. With underfill, the epoxy carries a substantial part of that load, and the joint life under thermal cycling can increase by an order of magnitude.

Underfill is also used to protect against mechanical shock and vibration, and to seal the gap against moisture and contamination. It is a reliability feature rather than an assembly step, and it should be specified because the product needs it rather than because it is available.

When Underfill Is Needed

The classic case is a large die on a package with a significant expansion mismatch, or a package mounted on a board with a very different coefficient of thermal expansion. Products that experience repeated thermal cycling, such as automotive electronics, power conversion equipment, and industrial controls, are the usual candidates.

Small packages with a close expansion match to the board often survive without underfill, and adding it costs time, material, and rework difficulty. The decision should be based on the thermal and mechanical profile of the product rather than on package size alone. A small package on a thick board that flexes in service may need it more than a large package on a rigid board that does not.

Portable products add a different driver: drop and bend resistance. Here the underfill’s contribution is mechanical rather than thermal, and the material properties that matter are different, with a lower modulus and higher elongation often preferred to absorb impact rather than resist it.

Materials and Flow

Underfill is a filled epoxy, and the filler content controls the expansion coefficient while also raising the viscosity. A higher filler loading brings the expansion closer to that of solder, which improves joint protection, but it also slows the flow and increases the risk of incomplete fill and voids.

Flow is driven by capillary action in a narrow gap, so the gap height, the surface energy of the board and package, and the temperature all affect how far and how fast the material travels. A larger gap flows more easily; a smaller gap protects the joint better but is harder to fill completely. The dispense pattern matters too, with an L-shaped or a single-line dispense along one or two edges being the usual starting point.

Preheating the assembly reduces viscosity and speeds the flow, but it also shortens the working time and can begin the cure prematurely. The temperature has to be high enough for flow and low enough to allow complete filling before the material gels, and that window is best established by experiment on the actual package.

The Dispense Process

Dispense volume is the parameter that most often decides success. Too little and the fillet does not form or the fill is incomplete. Too much and the material flows onto neighbouring components, wicks up the package side, or spreads to areas where it should not be. The correct volume is the one that produces a complete fill with a small, uniform fillet, verified by inspection.

Needle size, dispense speed, and the number of passes control the placement of the material. A faster pass that leaves a bead and lets the capillary action do the work is usually better than a slow pass that pushes material into one spot. Where the gap is very small, a two-sided dispense with a pause between passes lets the material flow before the second bead is applied.

Contamination control is critical. Flux residue and handling oils on the board change the surface energy and can stop the flow or create voids, so the board should be cleaned before underfill, particularly after a no-clean process where residue has been left in place. The cleaning method and the underfill material have to be compatible.

Voids, Fillets and Defects

Voids are the most common defect. They form when air cannot escape ahead of the flowing resin, when volatiles are released during cure, or when the flow front splits and rejoins around a feature. Voids reduce the mechanical coupling locally and can act as stress concentrators. They are reduced by slowing the flow, preheating to lower viscosity, using a vacuum-assisted dispense, and designing the dispense pattern so the front advances as a single even line.

Incomplete fill is a more serious version of the same problem. A gap that is only partially filled leaves joints unprotected, and the failure appears exactly where the fill stopped. Because the fill boundary is hidden under the package, X-ray or scanning acoustic microscopy is required to see it. Acoustic microscopy is the standard method for detecting voids, delamination, and incomplete fill, and it should be part of the qualification routine.

The fillet is the visible indicator of quality. It should be small, uniform, and continuous around the package, with a concave profile. A missing or uneven fillet suggests the volume or the placement is wrong, even if the fill underneath looks acceptable. Excess material that climbs the package side or spreads onto nearby parts indicates over-dispensing and should be corrected by reducing volume rather than by cleaning afterwards.

Rework and Inspection

Underfill makes rework difficult. Removing a package requires heating the assembly until the epoxy softens and then cleaning the site thoroughly, because residual underfill prevents a new package from seating properly and will interfere with a new dispense. The process is slow and risks damaging the pads and the laminate, and it should be treated as an exception rather than a routine repair.

Because of that, the decision to underfill should be made knowing that the package is effectively no longer reworkable. Where field repair or component replacement is likely, the alternative may be a different package, a stiffener, or a design change that reduces the thermal stress instead.

Inspection should combine acoustic microscopy for the hidden interface with visual inspection of the fillet and the surrounding area. Both are needed: the visible fillet confirms the dispense worked, and the acoustic image confirms the fill underneath is complete.

FAQ

Is underfill always needed for BGA packages? No. Many BGA assemblies run without it. It becomes necessary when thermal cycling, shock, or vibration would otherwise cause joint fatigue, which depends on the package, the board, and the product’s environment.

How do I check for incomplete fill? Scanning acoustic microscopy is the standard method because the fill interface is hidden under the package. X-ray can help with voids in some cases but does not show the resin.

Can underfilled packages be reworked? Yes, but it is slow and risky. The epoxy must be softened and the site fully cleaned before reassembly, and repeated rework is not practical.

Does the board need cleaning before underfill? Yes. Flux residue and oils change the surface energy and can stop the capillary flow or cause voids, so the surface must be clean and compatible with the resin.

What controls how far the underfill flows? Gap height, temperature, viscosity, filler content, and surface energy. The dispense volume and pattern then determine whether the fill is complete.

Conclusion

Underfill buys reliability by sharing thermal and mechanical load with the solder joints, and it is most valuable where packages and boards expand at very different rates or where the product sees shock and vibration. It also removes reworkability, so it should be specified deliberately, dispensed with controlled volume and temperature, and verified with acoustic microscopy. For the assembly topics around it, see our notes on PCB assembly, SMT assembly, quality management, and PCBA testing for how encapsulation decisions are made in 2026.