Underfill Capillary Flow Time and Void-Free Fillets

Underfill capillary flow is the mechanism that carries a low-viscosity resin under a package without pressure, and the time it takes decides how much of the gap is filled. Resin that arrives too late leaves an unfilled corner, and resin that flows too fast traps air that becomes a void at the die edge.

The flow is driven by surface energy and resisted by viscosity, so the variables that matter are the gap, the surfaces and the temperature rather than the dispense pressure. Controlling those three is the whole of the process.

How Capillary Flow Works

The resin wets the solder mask and the package underside and creeps into the gap, drawn by the difference in surface energy. Flow stops when the driving force balances the resistance, which is why an unfilled region is a wetting problem rather than a volume problem.

Flow time scales with the square of the gap in the ideal case, so a package stood off by 60 micrometres fills far more slowly than one at 100 micrometres. That quadratic relationship explains why a small change in solder ball collapse changes the process window sharply.

Gap Height and Flow Speed

The gap is set by the solder balls and by the collapse during reflow. Planarity of the pads, the weight of the package and the reflow profile all move the final stand-off, and the stand-off is what the underfill actually sees.

<img src="https://www.gopcba.com/wp-content/uploads/2021/05/section-3.jpg" alt="underfill dispensed along the edge of a BGA package” />

Measuring the stand-off on a sample assembly is therefore part of qualifying the process. A package that sits 20 micrometres lower than the qualification sample can double the flow time and leave an unfilled corner that only appears on a microsection. A BGA package with a large die and a small stand-off is the worst case, because the flow path is long and the gap is narrow at the same time.

Substrate and Solder Mask Effects

Surface energy differs between a bare copper pad, a coated finish and the solder mask, and the resin follows the higher-energy path. Where the mask overlap leaves a ridge beside the die, the flow front can be pinned by the step and stop short.

Plasma cleaning before dispensing raises the surface energy and makes the flow more repeatable. It also removes residues from the soldering process, which is why the flow time usually shortens after a plasma step is added, as discussed in the material on underfill processing. Mask thickness and the shape of the opening beside the package both change the local step, and a step of 20 micrometres is enough to slow the front.

Dispensing Pattern and Volume

The pattern decides whether the flow front arrives at the far edge as a straight line or as a curve. An L-shaped pattern along two edges gives a more even front than a single line along one edge, particularly on a square package.

The volume should be enough to fill the gap and leave a fillet of the specified height on each side. Dispense parameters and pattern selection are covered in the underfill dispensing guide, and the fillet is the part of the deposit that inspection can actually see. The pattern should be placed so that the front reaches all open edges at about the same time, which is why the dispense point is not always in the middle.

Board Temperature During Flow

Viscosity falls as temperature rises, so a board held at 70 °C fills faster than one at 25 °C. The substrate is normally heated to between 60 and 90 °C, and the temperature has to be controlled across the board rather than at a single point.

A board that is cooler at one edge produces a flow front that arrives late on that side. The result is a fillet that looks acceptable at the edges and an unfilled region under the package that no optical inspection can find. Temperature should be held within a few degrees across the board, and a fixture that heats locally will produce a fillet that is short on the far side.

Voids and Their Causes

Voids come from three sources: air trapped by an irregular flow front, moisture released from the board or the package, and outgassing from the resin itself during cure. Each leaves a different distribution, so the pattern of voids points to the cause.

Voids at one corner suggest a flow problem, voids spread evenly under the die suggest moisture, and voids concentrated near the dispense path suggest that air was drawn in at the start of the flow. The relationship between the fill and package reliability is treated in the material on board level reliability. A slow, even flow front is more likely to avoid voids than a fast one, because the air ahead of the front has time to escape along the open edges.

Cure Profile After Flow

Cure follows the flow and is usually run in two stages: a short dwell to let the resin settle, followed by the main cure. The dwell matters because a resin that is cured while still moving keeps the stress of the flow frozen into the fillet.

The cure schedule should be measured on the product rather than taken from the datasheet. A large board takes longer to reach temperature than a coupon, and the difference shows up as a fillet that is soft in the middle of the panel.

Inspection of the Fillet

Inspection looks at the fillet height, its continuity around the package and any visible voids near the edge. A fillet that is missing along one side is the visible evidence of a flow that did not travel as far as it should.

Bulk voids under the die require an acoustic scan, and the acceptance limit is normally expressed as a percentage of the area under the die rather than as a count. Setting that limit before production avoids the argument that follows the first scan result. A fillet measured at three points around the package gives a better picture than a single height, because the flow is not symmetrical.

Records and Process Limits

Records should carry the resin lot, the board temperature, the dispense pattern and volume, the flow time observed and the cure profile used. gopcb keeps those values with the lot so a change in flow time can be tied to a material or a temperature shift.

Process limits should state the allowed fillet height, the maximum void area and the maximum flow time. A flow time limit is a useful control, because it moves before the void area does and gives warning while the process is still producing acceptable parts. Where the resin is changed, the flow time should be re-established before the new lot is released, even if the chemistry is nominally equivalent.

FAQ

What controls underfill capillary flow time? Gap height, resin viscosity and the surface energy of the surfaces the front travels over, with gap height having the strongest effect because flow time scales with its square.

Why is a corner of the package left unfilled? Either the flow front was pinned by a mask step or the board was cooler on that side, and both reduce the distance the resin travels before it gels.

Can voids be removed by dispensing more resin? No. Additional volume raises the fillet, while trapped air stays where the flow front left it, so voids are addressed through temperature, plasma treatment and pattern.