Underfill Process Guide for PCB Assembly

Underfill is a filled polymer that is dispensed beside a package and drawn under it by capillary action, where it locks the solder joints to the board and spreads the strain that thermal cycling would otherwise concentrate on the outermost balls. It is used on large ball grid arrays, on chip scale packages and on any device where the stand off is small relative to the thermal mismatch between the package and the board.

Why Underfill Is Used

A solder joint carries two loads at once: the thermal expansion of the package and that of the board, which differ by a factor that depends on the materials. Each temperature cycle strains the joint, and the outermost joints of a large package see the greatest displacement because they are furthest from the neutral point.

Underfill changes that picture by mechanically coupling the package to the board over its whole area. The strain is distributed across the filled volume instead of being concentrated at the joints, and the fatigue life of the outer row improves by a large factor, which is what allows a large die on a thin package to survive field conditions.

The process is not free. It adds a dispense and cure step, it is difficult to rework, and a poorly applied underfill can trap voids that become the new failure site. It is used where the reliability requirement justifies it rather than as a general purpose improvement.

Material and Filler Content

An underfill is an epoxy or a similar resin loaded with silica filler, and the filler content controls the coefficient of thermal expansion. A higher filler content brings the expansion closer to that of the solder and the package, but it also raises the viscosity and slows the flow.

The flow behaviour is described by capillary flow, in which the material is drawn into the gap by surface tension. The gap height, the filler particle size and the wetting of the surfaces all affect the speed, and a gap that is narrower than a few times the largest particle will filter the filler out and leave a resin rich region with different properties.

Filler settling is a storage issue. A material left standing will separate, and the dispensed ratio of resin to filler will then change through the shift. The container should be conditioned and mixed according to the supplier instruction, and the batch should be recorded so that a flow change can be traced.

Dispense Patterns

The pattern determines how the material fills the gap. A single line along one edge is simple but leaves the far corners to be filled last, a J pattern spreads the entry points, and a cross or L pattern is used where the flow path is long. The pattern should be chosen from the package size and the gap height.

The volume must be enough to fill the gap and form a small fillet around the package edge without flowing onto neighbouring parts. Under dispense leaves an incomplete fillet and exposed joints at one side, while over dispense wets the wrong areas and can wick under adjacent components.

Dispense temperature is a controlled variable because viscosity falls as the temperature rises. A needle that is warm flows faster, and a needle that has been idle will be cooler and slower, so a warm up before the first board of a run avoids a first article that does not match the rest of the run.

Flow, Wetting and Voids

Voids are the dominant defect. They come from air displaced ahead of the flow that cannot escape, from moisture released from the board or the package during cure, and from flux residue that decomposes under the package and generates gas that has nowhere to go.

The escape route for trapped air is normally along the flow front, so the last area to fill is the most likely to contain a void. Dispensing from one edge and letting the flow travel in one direction gives the air a path, while dispensing from several sides at once traps it in the middle.

Moisture is managed by baking the assembly before underfill, and by choosing a material and a cure schedule that release the gas slowly. The bake must be compatible with the components, and the interval between the bake and the dispense must be controlled, because a board that is left in a humid room reabsorbs moisture.

Cure and Process Control

The cure converts the dispensed liquid into a solid with the intended properties. A partial cure leaves the material soft, which reduces the mechanical coupling, while an excessive cure temperature stresses the package and can crack the die or the joints through expansion of trapped volatiles.

The cure profile should be measured at the package, not at the oven set point, and the measurement should be made on a board that carries the same thermal mass. The flow and the cure are linked: a snap cure that begins before the flow is complete will freeze the material in place and leave the far corners empty.

Process control is built on a small number of measurements. The dispensed weight, the fillet width on two opposite edges, the flow time and a periodic section or acoustic scan give a picture that a visual check alone cannot provide.

Inspection and Reliability

Inspection of an underfilled package is difficult from above, and the useful techniques are acoustic microscopy, which images the interface through the package, and a section that shows the filler distribution and the fillet. A visual check confirms the fillet but says nothing about what lies underneath.

The reliability question is answered by thermal cycling, and the result depends on the void content, the filler distribution and the adhesion of the material to the die, the package and the board. A test vehicle should represent the worst case package size and the thinnest board in the product family.

Where a failure is found, the analysis should begin with the interface rather than with the solder. A delamination at the package surface and a void in the middle of the gap produce different signatures, and both are more common than a fatigue crack in the joint itself.

Practical Rules

Choose the filler for the thermal expansion match, the gap for the flow, and the pattern for the geometry. Dispense from a single edge where the flow path allows it, bake before the dispense, and cure on a measured profile rather than a set point.

Record the material batch, the dispense weight, the flow time and the inspection results with the build records and the defect history, and apply the same discipline to underfill as to the inspection of the joint itself.

FAQ

What does underfill actually do? It couples the package to the board over its whole area, so thermal strain is shared rather than concentrated on the outermost solder joints.

Why does underfill form voids? Air displaced ahead of the flow, moisture released during cure and gas from flux residue under the package. The last area to fill is the most likely site.

Why is rework so difficult? The material fills the gap and bonds to the package and the board. Removal usually damages the package or the solder joints, so the process is designed to be right the first time.