Underfill for Board Level Reliability: When It Pays Off

Underfill is a filled epoxy that is dispensed alongside a package and drawn under it by capillary action. It exists for one reason: to spread the stress that thermal expansion puts on solder joints across a larger volume of material. Used in the right place it is transformative, and used everywhere it is an expensive habit.

The Problem It Solves

A package and a board expand at different rates when the assembly is heated. The solder joints must absorb the difference, and the amount they must absorb grows with the package size, because the distance from the neutral point to the outermost joint grows with it.

The joint is a small piece of metal with a limited capacity for plastic deformation before it cracks. Once the difference in expansion exceeds that capacity, every thermal cycle consumes part of the life of the joint.

Underfill changes the mechanics by coupling the package to the board through a stiff material. The joints no longer carry the whole of the differential movement, because the underfill and the package now act together as a composite.

When It Is Justified

The classical candidates are large area ball grid arrays, chip scale packages with a large body relative to the joint pitch, and assemblies that must survive many thermal cycles in a wide temperature range.

The volume production case is a consumer product exposed to repeated cycling through use, where a large package on a thin board would otherwise crack. The reliability case is an automotive or industrial product with a specified cycle count and a wide range.

Where the package is small, the board is thick and the environment is benign, the same money spent on a thicker board or a better via structure may buy more reliability. The decision belongs to the reliability calculation rather than to style.

Capillary Flow and What Controls It

The underfill must reach the centre of the package without leaving voids. The flow is driven by capillary pressure and resisted by viscosity, so the gap height, the filler particle size and the temperature of the board all matter.

A small gap with large filler particles will not fill completely, and the resulting void is a stress concentration that is worse than no underfill at all. The particle size must be small relative to the gap, and the gap is set by the solder joint height after reflow.

Dispensing pattern matters as much as the material. A single line along one edge produces a flow front that arrives at the far edge at different times, which traps air at the last point to fill. A pattern with multiple segments, or an L shape, produces a more even front.

Process Sequence and the Curing Step

Underfill is applied after reflow and after any cleaning step that the assembly requires, since the filleting around the package would otherwise be disturbed. The board is usually preheated to reduce viscosity and accelerate the flow.

The dispensed volume is calibrated so that the flow reaches the far edge and forms a small fillet, but does not spread onto neighbouring components. The fillet size is a useful visual indicator that the fill is complete.

Curing follows the material specification, which typically requires a period at an elevated temperature. The cure must complete, because an underfill that has not fully cured is soft, and soft underfill provides much less of the benefit that justified its use.

Inspection and What Can Go Wrong

The two defects that matter are voids and incomplete fill. Both are best detected by scanning acoustic microscopy, which images the interface between the underfill and the package and reveals areas where the two are not in contact.

The third defect is delamination of the underfill from the board or the package after thermal cycling. A delaminated region no longer transfers load, and the joints under it return to carrying the full differential movement.

Our inspection notes describe which techniques are applicable to a package after underfill has been applied, and why X-ray becomes less informative once the fill is in place.

Rework and the Cost of the Decision

Underfilled packages are difficult to remove. The underfill must be heated and mechanically removed, which risks damaging the pads and the neighbouring components, and the package itself cannot be reused.

The consequence is that the decision to underfill is also a decision about rework. A design in which a critical package cannot be replaced is a design that will be scrapped rather than repaired when something else on the board fails.

Where rework access is important, the alternatives are a cornerbond, in which the material is applied only at the corners, or a reworkable underfill formulated to soften at a temperature that permits removal. Our reballing notes describe how a removed package is returned to service.

The Related Question of Board Design

The reliability that underfill provides can also be obtained by reducing the stress in the first place. A board with a coefficient of thermal expansion closer to that of the package, or a package with a compliant joint structure, needs less help.

Vias placed under the package to control the local expansion, and a board thickness that resists bending, both reduce the demand on the joints. Our component reliability notes describe how the thermal mismatch is estimated.

Where the design already includes these measures, the underfill may be unnecessary, and a thermal cycling test on a coupon is the way to demonstrate that rather than assume it.

Qualifying the Process

The qualification consists of a thermal cycling test on a daisy chained package, with resistance measured continuously, run until failures appear. The result is a characteristic life that can be compared against the requirement.

The test should use the production underfill, the production dispense pattern and the production cure, because all three affect the outcome. A test with a laboratory dispense pattern predicts a lifetime that the line will not reproduce.

Our solder defects notes describe the failure signatures that appear when the joints are the limiting element, which is the comparison that shows whether the underfill helped.

Process Control and Verification

On a design of this kind, capillary flow is the item that decides how the rest of the board is arranged. The process window is set by the narrowest step in the flow, so an improvement anywhere else shows up as margin rather than as yield until that step is addressed. A short note on the drawing about handling, storage or packaging is often worth more than an extra decimal place on a tolerance.

FAQ

Does every ball grid array need underfill? No. Small packages on thick boards frequently pass the required cycle count without it, and the thermal cycling test is what distinguishes the two cases.

Can underfill be applied after assembly is complete? It is applied after reflow but before the final assembly steps, and it is generally incompatible with later rework of the package it protects.

What does gopcb provide for an underfilled assembly? We provide dispense pattern development with flow front verification, cure records, scanning acoustic microscopy of the fill, thermal cycling of daisy chained packages and the resulting lifetime data. Where the calculation shows the joints have margin, we say the underfill is not needed.