Underfill: Preparation, Placement and Process Control

Underfill is the material that is drawn beneath an area array package to distribute the strain between the solder joints and the board. It is applied as a bead along the edge of the package and then flows by capillary action until the gap is filled. The dispense pattern is what decides whether that flow reaches every corner or leaves a void on the far side. Underfill is therefore a flow process with a dispense step attached, and the pattern is the parameter that controls it.

What Underfill Does

A large package and a rigid board expand at different rates, so the corner joints of an area array package carry the highest strain when the assembly is thermally cycled. Underfill mechanically couples the package to the board and spreads that strain over the whole joint rather than concentrating it at the corner.

The result is a large improvement in thermal fatigue life for the joints. The cost is an extra process step, a material that has to be cured, and a package that can no longer be reworked without significant effort. That irreversibility is the reason the underfill decision belongs to the reliability analysis rather than to the line.

Flow and Capillary Action

Capillary flow is driven by the pressure difference across the liquid front, which depends on the surface energy and on the gap height. A smaller gap produces a stronger capillary force but also a higher resistance to flow, so the flow time rises sharply as the standoff height falls. A low standoff package therefore needs a warmer dispense and a longer flow time than a package with a taller gap.

Temperature affects the picture from both sides, because it lowers the viscosity and increases the flow speed. The dispense temperature is therefore chosen to balance the flow against the cure behaviour of the material. The substrate temperature matters as much as the material temperature, because it is the board that heats the material as it flows.

Dispense Patterns

The simplest pattern is a single line along one edge, which relies on the material flowing the full width of the package. It works for a small package with a generous gap and fails on a large one because the flow front becomes uneven.

Patterns with lines along two adjacent edges or along all four are used for larger packages, so that the flow front meets in the middle rather than travelling the full distance. Meeting in the middle is preferable because any trapped air is pushed towards the edge rather than trapped in the centre. The pattern is chosen from the package size and the gap height rather than from habit. The pattern should also be recorded in the process instruction, because an operator who improvises will produce a different fill on every shift.

<img src="https://www.gopcba.com/wp-content/uploads/2025/05/办公司风采.jpg" alt="Cross section showing underfill fillet beneath a package” />

Fillet Control

A controlled fillet is needed for reliability, because it transfers the load from the package edge into the board and prevents the material from cracking at the interface. A fillet that is too small provides little reinforcement, and one that is too large can wick onto neighbouring components. A fillet that climbs onto a neighbouring component is a cosmetic defect and a reliability risk at the same time.

The fillet size is set by the dispensed volume and by the flow time before cure begins. Dispensing a little extra and allowing the flow to develop gives a more consistent result than trying to hit an exact volume. The fillet size should be checked on a cross section rather than judged from the appearance of the bead.

Voids and Their Causes

Voids form when the flow front traps air or when a volatile is released before the material gels. The common causes are an uneven flow front, a dispense that lets the material enter from both sides at once and a gap that has not been dried after cleaning. The gap geometry also matters, because a standoff that varies across a package produces an uneven flow front on its own.

Moisture in the gap is a frequent contributor, because the cleaning process leaves water under the package. A bake before underfilling removes it, and the bake is often the difference between a void free result and a rejected one. Our inspection guide covers the methods used to find voids. Where the void criterion is specified by the customer, the measurement method should be agreed as well as the limit.

Cure and Its Interaction

The cure is normally a two stage schedule, with a moderate temperature period to allow the flow to complete and then a higher temperature to cure the material. Starting the cure too early stops the flow and freezes an incomplete fill in place. The transition from flow to cure is the single most important timing in the process, and it should be verified with a profile.

The cure also generates heat. A large volume of material in a thick section can exotherm, and the temperature inside the material may be much higher than the oven setting, which affects the properties of the cured resin. Where the volume of material is large, a slower cure at a lower temperature is usually safer than a fast one.

Inspection and Verification

Optical inspection can see the fillet but not the fill underneath. X-ray with a suitable contrast can reveal voids and an incomplete flow front, and scanning acoustic microscopy is used where a detailed picture of the interface is required.

A destructive cross section remains the definitive check, and it should be used on the first article and after any change to the material or the pattern. The section is also the best way to measure the fillet and to confirm that the fill reached the far edge. Our quality documentation describes how these results are classified at gopcb.

Common Problems

The common problems are an incomplete fill on the far edge, voids near the centre, an uneven fillet, material on adjacent components and delamination after thermal cycling. Each of them points at a different variable in the process.

An incomplete fill on the far edge is almost always a flow time problem, while voids near the centre are usually a dispense pattern or a moisture problem. Our solder defects guide describes how these conditions are classified. The position of the defect is the diagnostic, and it usually points to one variable rather than to several.

Process Control Points

The controls are the material lot and its thaw procedure, the bake before dispense, the dispense temperature and pattern, the flow time before cure, the cure profile and the inspection result on a sample.

Every one of those is recorded per lot, and the first article section is the evidence that the pattern and the flow time are correct for the package. A pattern that was validated on one package size should not be transferred to another without repeating that check. Underfill is a process with a narrow window, and the window moves with the package geometry.

FAQ

Why does the underfill not reach the far edge? Usually because the flow time was too short or the dispense pattern only feeds one edge. A larger package normally needs feed from two or four sides.

What causes voids in underfill? Air trapped by an uneven flow front and moisture released from the board during cure. Baking before dispense removes the moisture contribution.

Can an underfilled package be reworked? Only with difficulty, because the material has to be removed from beneath the package without damaging the board. This is one reason the underfill decision is made early.