Underfill Flow Control: Filling the Gap Under a Package

Underfill is a filled polymer dispensed along the edge of a package and drawn into the gap beneath it by capillary action, where it cures and couples the package, the solder joints and the board into one structure. Its purpose is mechanical, because it reduces the strain that thermal cycling places on the joints around the perimeter of a large package.

Getting the material into the gap is what decides whether the work was worth doing. An incomplete fill leaves some joints unsupported and concentrates strain on the joints that were filled, which in the worst case is a poorer condition than no underfill at all. The fill is a process result rather than a specification, and it is confirmed by a cross section on a sample.

What Underfill Does

The coefficient of thermal expansion of a package differs from that of the board, so a temperature change moves the two by different amounts. The solder joints absorb that difference, and the joints at the perimeter of a large package see the largest displacement. A larger package and a larger temperature excursion both increase that displacement.

Underfill spreads the load between the joints and the cured polymer, and it transfers part of the stress into the package and the board. The mechanism is mechanical rather than electrical, and it changes the failure mode rather than preventing stress from existing.

Capillary Flow and the Gap

Flow through the gap is driven by surface tension and opposed by viscosity. A narrower gap gives a stronger driving force and a greater resistance at the same time, and the balance between the two sets the flow speed. A lower viscosity helps, and the filler content that makes the cured material stiff also raises its viscosity.

The gap height is set by the joint height, which is set by the solder volume and the weight of the package. A gap smaller than the filler particle size in the underfill blocks the flow entirely, which is why particle size is chosen against the minimum gap on the product.

Dispense Patterns

The pattern is a line, an L or a U, and it is chosen to fill the gap without trapping air. Material flows most reliably when it advances as a single front, and a pattern that produces several fronts meeting under the package traps air where they join. The pattern is also constrained by the components around the package, which limit where a needle can reach.

The dispense volume has to be enough to complete the fill and to form a fillet without flowing onto neighbouring components. Volume is calculated from the gap and the footprint and then corrected by experiment, because wetting and pattern efficiency are not predictable from geometry alone.

Substrate Temperature and Flow Rate

Warming the board lowers the viscosity and speeds the flow. The temperature is set to a value that gives a flow time consistent with the dispensing equipment and does not begin the cure, and it is measured on the board rather than inferred from the stage setting. A stage that is hotter at the centre than at the edge gives a fill that advances unevenly.

The flow rate is slow by design. A large package can take minutes to fill, and the process is often run with the board on a heated stage while the material advances. Shortening that time by raising the temperature risks curing the material before it reaches the far edge.

Wetting and Surface Condition

Flow depends on the material wetting the surfaces it travels over. Flux residue, solder mask contamination and moisture all change the surface energy, and a surface that repels the underfill produces an incomplete fill that looks like a volume problem.

Cleaning before dispensing is therefore part of the process rather than an optional step. Where a no-clean flux is used, its residue has to be compatible with the underfill, and the combination is verified once and then held under change control. The preparation side is described in the notes on surface preparation before coating. A change of flux is therefore a change of underfill process, even when nothing else on the line has moved.

Fillet Formation

The fillet is the material remaining around the perimeter, and it has to be continuous and of a controlled size. A fillet that is too small points to an incomplete fill, and one that is too large can reach a neighbouring component or a test point. A fillet that varies around the perimeter shows that the flow front did not advance evenly.

Fillet height and width are measured on a sample after cure. That measurement is a proxy for the fill in the middle of the package, which cannot be seen, and it is a good proxy only while the process is stable.

Voiding and Its Causes

Voids in the cured underfill come from three sources: air trapped by a flow front, moisture driven out of the board during cure, and shrinkage during polymerisation. The first is a dispense pattern problem, and the other two are a temperature and a storage problem. Moisture is largely a storage question, and the remedy is a controlled environment rather than a process change.

Their effect is local. A void above a joint removes the support that the joint was given, and the resulting strain concentration is what makes an otherwise acceptable fill ineffective. Measurement follows the same practice used for other encapsulated volumes.

Cure Schedule and Verification

The cure schedule is taken from the material data and then verified on the product, because the board and the components absorb heat and slow the real cure. A cure that is incomplete leaves the material soft, and a soft underfill provides much less support than a fully cured one. The difference between the two is large enough to change the mechanical result rather than only the appearance.

Verification is by a thermal measurement, by a hardness check or by a cross section. A cross section shows fill, voids and cure together, and the pattern of the schedule follows the same discipline as any other coating, as described in the notes on conformal coating cure schedules.

Inspection and Records

Inspection covers the fillet, the flow marks and the presence of material where it should not be. X-ray shows voids inside the cured material on the areas it can penetrate and does not show flow defects, so the visual and the radiographic results answer different questions. Together they cover the surface and the interior, and neither is a substitute for the other.

The records hold the material and its lot, the dispense pattern, the volume, the stage temperature, the flow time and the cure schedule. A change to any of those fields is a process change, and the joint defects that underfill is meant to prevent are described in the notes on voids in BGA joints.

FAQ

Can underfill be dispensed after rework? Yes, with a different pattern and a lower volume, because the reworked area has a different surface. Rework under a filled package usually requires removing the old material first.

Does a smaller gap fill faster? The driving force rises and the resistance rises faster. Flow speed falls as the gap narrows, and a gap below the filler particle size blocks it.

Is a visible fillet proof of a complete fill? No. It is a proxy. A cross section on a sample is what confirms the middle of the package.