Underfill And Corner Bonding For BGA Packages

Underfill is an adhesive that is dispensed along one edge of a ball grid array after reflow and allowed to flow by capillary action until it fills the space between the package and the board. It reduces the strain that thermal cycling puts on the solder balls, and for a large package on a thin board it is often the difference between a product that survives its environment and one that does not.

This article covers what underfill does, how the dispense pattern is chosen, how voids and fillet shape are controlled, and when a corner bond is enough.

Why Underfill Helps

The solder balls of a grid array are short and stiff, and the package and the board expand by different amounts as the assembly heats. The differential movement has to be absorbed by the balls, and because they are short, the strain per unit length is high. Underfill changes the load path: the cured adhesive bonds the package to the board across the whole area beneath it, so the differential movement is distributed through the adhesive instead of being concentrated in the balls.

The benefit is greatest where the package is large, where the board is thin, and where the temperature excursion is wide. It is smallest where the package is small and the board is thick, because the balls are then able to absorb the movement without reaching their fatigue limit. The decision is made by comparing the calculated strain with the fatigue data for the ball size and alloy in use, not by habit, and the environment that sets the excursion is the same one that drives the test sequence described under potting and dispensing adhesives.

Choosing The Dispense Pattern

The adhesive is dispensed along one or more edges of the package and flows under it by capillary action. A single edge, usually the longest one, gives the shortest flow path and the fewest voids, and it is the first choice wherever there is room to dispense. Where the flow path is too long for the adhesive to travel before it gels, a second edge is added, normally the adjacent one, so that the two fronts meet at a corner.

The pattern that causes the most trouble is the one that dispenses along opposite edges at once. The two flow fronts meet in the middle, and any air they trap between them stays there, because there is no free edge for it to escape through. Filling along one edge and letting the air leave through the others is the arrangement that produces the cleanest fill. Where the package is surrounded by components, the dispense path has to be kept clear, and the keep-out around the package becomes part of the layout rather than an assembly detail, in the same way as the clearances described under placement order and pad positioning.

Controlling Voids

A void under a package is a place where the adhesive did not reach, and it leaves the balls in that area without the support the rest of the array receives. Voids form for three reasons: air trapped by a flow front that met another front, air drawn in when the adhesive advances faster along one side than the other, and moisture released from the board or the package during cure.

The first two are controlled by the dispense pattern and by the temperature of the board. Warming the assembly reduces the viscosity and speeds the flow, which shortens the time available for air to escape in some cases and improves it in others, so the temperature is established by experiment on a real package rather than from a data sheet. The third is controlled by baking the board before assembly if it has absorbed moisture. Inspection is done with an acoustic microscope, which images the adhesive and the air by their difference in acoustic impedance. The same imaging discipline is used in the quality characteristics of a board design.

Cure, Fillet And Material Choice

The cured adhesive must be stiff enough to carry the load but not so stiff that it transfers stress into the package and the solder mask. A rigid underfill with a high modulus below its glass transition and a low expansion coefficient protects the balls, while a flexible one deforms with the assembly and offers less benefit. The expansion coefficient matters because the adhesive has to match the board rather than the package if the goal is to reduce the differential movement.

The fillet is the visible result of the process. It should form a continuous concave fillet around the package, roughly a millimetre wide, with no gaps and no adhesive climbing the side of the package. A fillet that is too small means too little adhesive was dispensed and the fill under the package may be incomplete. A fillet that is missing on one side means the flow went the other way, and the void is probably on the side with no fillet.

Corner Bond As A Lighter Option

A corner bond applies a small dot or a short line of adhesive at the corners of the package instead of filling the whole area beneath it. It does not remove the differential movement, so the balls still carry the strain, but it reinforces the corners where the strain is highest and where a crack usually starts. The process is much faster, it needs no capillary flow and therefore creates no voids, and the joint remains reworkable with local heat.

The trade is that the protection is partial. A corner bond helps against a specific failure mode, the corner ball cracking under a moderate excursion, and does nothing for a large package on a thin board where the whole array is loaded. It is also sensitive to placement: a dot that lands on the mask instead of on the board or the package edge contributes nothing, so the geometry has to be defined and inspected rather than left to the operator.

Process Control

Control of an underfill process rests on three measurements. The weight or volume dispensed is recorded for each unit, because the amount dispensed is the main determinant of whether the fill completes. The fillet is inspected visually for continuity and width. And the void content is checked on a sample by acoustic imaging, with an acceptance limit expressed as a percentage of the area beneath the package and as a maximum size for a single void.

Where the assembly will later be coated, the underfill and the coating have to be compatible, because a coating that does not adhere to the cured adhesive will lift at the boundary and take the protection with it. The coating requirements that the joint has to survive are described under conformal coating as board protection, and the combination of the two materials should be qualified together rather than chosen separately.

FAQ

Does underfill make the assembly unrepairable? It makes rework of the package much harder, because the adhesive has to be softened and removed before the package can be lifted. Where repair is expected, a corner bond or an edge bond keeps the option open at the cost of less protection.

How is the void content measured? Scanning acoustic microscopy images the interface through the board or the package and shows the adhesive and the air by their difference in acoustic impedance. The result is reported as a percentage of the area beneath the package.

Can underfill be applied before reflow? Some processes dispense before placement so that the adhesive is cured together with the solder, but the joint quality is then harder to inspect and the adhesive must not interfere with the solder. For most assemblies the adhesive is applied after reflow.