Underfill Flow and Cure Control for Area Array Packages

Underfill is a filled epoxy that flows by capillary flow into the gap under an area array package and then cures to a rigid layer that shares the stress between the package and the board. It is applied after reflow and before the assembly is considered finished, and the flow and the cure are the two stages where it succeeds or fails.

What Underfill Does

The thermal expansion mismatch between a package and the board concentrates strain in the solder joints at the perimeter of an array, and repeated thermal cycling eventually cracks them. Underfill spreads that strain across the whole interface, so the joints see less movement and the assembly survives more cycles.

It also changes the failure mode. An underfilled assembly usually fails by a crack in the underfill or at its interface rather than by a cracked joint, which is a slower and more predictable mechanism. That is the reason underfill is used in products where field replacement is expensive.

Capillary Flow and Gap Height

The underfill flows into the gap between the package and the board by capillary action, which depends on the gap height, the wetting of both surfaces and the viscosity of the material. A gap of tens of microns, typical for a ball array, creates a strong capillary force but also a high resistance to flow along the length of the package.

Because the resistance rises with distance, flow time increases sharply with package size. A large package may need several minutes to fill, and the material must stay fluid for that entire period. Where the flow stops before the far edge is reached, the result is a void at the last corner to fill, which is exactly where the strain is highest.

Dispense Pattern and Volume

The dispense pattern determines how the flow front travels, and the aim is a single, continuous front that reaches the far edge without trapping air. Common patterns are an L along two edges, an I along one edge, or a line along one edge with a second pass, and the choice depends on the package shape and the number of sides that are accessible.

Volume is calculated from the gap, the package area and the desired fillet, with an allowance for what stays on the board outside the package. Too little volume leaves an incomplete fill; too much produces a large fillet that can wick onto neighbouring components or onto the board surface where it interferes with later processes.

Substrate and Flux Compatibility

The flow depends on the surfaces it travels over, and flux residue is the most common obstacle. A residue that repels the underfill stops the front or splits it, and the defect appears as a void or an unfilled region even though the volume was correct. Cleaning before underfill, or selecting a flux that is compatible with it, is the practical control.

Surface energy matters on both sides of the gap. A board surface that has been plasma treated wets more readily and flows faster, while a solder mask with a low surface energy can slow the front along one edge. Where flow is inconsistent between lots of boards, the mask or the treatment is worth checking.

Voids and Their Causes

Voids in underfill come from three sources: air trapped when the front splits and rejoins, solvent or moisture released during cure, and incomplete filling at the far corner. The first appears as isolated bubbles along the flow path, the second as a diffuse foam, and the third as a single large void at the end of the flow.

Diagnosis starts with the pattern. A void at the far corner is a flow problem, a bubble in the middle is a front-merging problem, and bubbles throughout the material point to the cure or to the material itself. The inspection method, usually acoustic or X-ray, has to be able to distinguish them, because the corrections differ.

Cure Schedule and Glass Transition

The cure converts the flowing resin into a rigid solid, and the schedule is defined by the material: typically a period at an elevated temperature, with the glass transition temperature of the cured material as the key property. A partially cured underfill has a lower modulus and provides less protection, and the deficiency is invisible on the board.

Cure also generates stress. A material that cures too quickly or at too high a temperature can warp the assembly or pull the package, and a two-stage cure is often used to let the material gel before the final hold. The schedule should be verified with the material supplier’s data and with a measurement rather than with a clock.

Inspection After Underfill

Inspection looks for the fillet, for voids and for contamination outside the package. The fillet should be continuous and of consistent height around the perimeter, and a missing fillet indicates an incomplete fill even when the interior looks sound. Void inspection usually requires an acoustic or X-ray method, since the underfill hides the joints it protects.

Because underfill is opaque, the joints beneath it can no longer be inspected optically or by X-ray in the usual way, and the joint quality has to be established before the underfill is applied. That makes the pre-underfill inspection the last opportunity to reject a bad joint, and it should be treated as a gate rather than a formality.

Rework Implications

Rework after underfill is difficult and often impractical. Removing a package requires heating the underfill to soften it and then mechanically separating it, which stresses the surrounding area, and the site has to be cleaned of every trace before a replacement part and new underfill can be applied.

The practical consequence is that the process is designed to avoid rework. Where rework is expected, the product is usually assembled without underfill or with a removable material, and that decision belongs in the design rather than in the process.

Process Control and Records

The parameters that matter are the dispense pattern, the volume, the substrate temperature during flow, the flow time allowed, and the cure schedule. Substrate heating is often used to lower the viscosity and to speed the flow, and the temperature has to be stable because a change of a few degrees alters the flow time measurably.

Recording the flow time and the resulting fillet for each lot gives an early indication that the material, the gap or the surface has changed. A flow time that has crept upward over a month usually precedes a fill defect, and catching it at that stage is a material change rather than a rework exercise.

Related reading: our fabrication notes, board quality and design release notes cover the same ground.

FAQ

Why is underfill used under a BGA? To spread the strain caused by thermal expansion mismatch across the whole interface, so the perimeter joints see less movement and survive more thermal cycles.

What causes a large void at one corner of the underfill? Incomplete filling. The flow front stops before reaching the far corner, usually because of flux residue, a low substrate temperature or insufficient volume.

Can an underfilled package be reworked? It is difficult and often impractical. The site must be fully cleaned before a replacement part and new underfill, so the process is designed to avoid rework.