Die Attach Materials and Void Control in Package Assembly

Inside a package, the die is held in place and cooled by a layer that is often only tens of micrometres thick. That layer has to conduct heat, survive thermal cycling, keep out moisture and stay mechanically stable for the life of the product. Void content in the attach layer is the single most common reason it fails to do all four at once.

What the Attach Layer Has to Do

The attach layer performs four jobs simultaneously. It fixes the die in position so that wire bonding and moulding can proceed, it conducts heat from the junction into the substrate, it provides an electrical connection where that is required, and it absorbs the mechanical and thermal mismatch between two materials with very different expansion rates.

Those requirements pull in different directions. A material that is highly filled for thermal conductivity becomes stiff and brittle, which increases stress on the die. A compliant material relieves stress but conducts less heat. The choice is therefore a balance that depends on the power density and the expected temperature excursion.

Material Families

Epoxy based attaches are the workhorse for low and medium power devices. They adhere well, cure at moderate temperature and can be filled with silver for conductivity. They are also the easiest to dispense and rework, and they tolerate the coefficient of thermal expansion mismatch reasonably well when the die is small.

Solders are used where the thermal path must be short and the electrical connection is part of the design. Gold tin and tin silver alloys wet the backside metallisation and form a metallurgical bond that resists fatigue better than a polymer. Silver sintering produces a bond with very high conductivity and a melting point well above any operating temperature, at the cost of pressure and a longer process.

Void Formation in the Attach Layer

Voids form wherever gas cannot escape before the material sets. Trapped air from a dispensed pattern, solvent and volatiles released during cure, and flux residues in a soldered attach all produce them. The pattern in which the material is dispensed is one of the strongest influences, because a closed pattern traps air in the middle as the die settles.

Voids matter because they block heat. A void directly under the junction creates a local hot spot, and the temperature rise in that region can be far larger than the average across the die. The effect grows with power density, which is why the acceptable void content falls as devices get smaller and more powerful.

Thermal Performance and Conductivity

The attach layer contributes to the total thermal resistance along with the die, the substrate and the interface to the heat sink. Its contribution is the thickness divided by the conductivity and the area, so both a thick layer and a low conductivity material raise the junction temperature. Voids act as local areas of near zero conductivity.

Thickness is set by the dispensed volume, the die placement force and the bond line the process achieves. A thinner layer is better thermally, but it must still be continuous, because a layer thin enough to touch the die and the substrate unevenly is worse than a slightly thicker one that is uniform. The overall thermal path is discussed further in this guide to PCB thermal management.

Dispensing, Stamping and Placement

Dispensing is the most flexible method and the one most sensitive to pattern. A dot, a cross or a star pattern is chosen to let material flow outward as the die is pressed down, pushing air ahead of the front rather than trapping it. The volume is calculated from the target bond line, the die area and a small allowance for the fillet at the die edge.

Stamping and printing suit high volume production with consistent die sizes, because they deposit a well defined pattern very repeatably. Placement force and dwell time then control the final bond line, and both need to be set so that the material spreads to the edges without flowing onto the wire bond pads or over the die top.

Cure, Reflow and Process Windows

Polymer attaches cure either at elevated temperature or with a snap cure that develops strength quickly. The profile has to bring the material to the required degree of cure without generating so much stress that the die cracks, and it has to drive off volatiles early enough that they do not form voids under the die. A staged profile usually gives the best result.

Sintered and soldered attaches follow a different path, because the bond forms by diffusion or by melting rather than by polymerisation. For sintering, temperature, pressure and time together determine the density of the bond, and a small change to any of them shows up as a change in void content. The windows are narrower, so process control has to be tighter.

Bleed, Outgassing and Contamination

Some polymer systems separate during cure and release a low viscosity fraction that migrates outward from under the die. This bleed can reach the wire bond pads, where it prevents a reliable bond from forming, and it can also carry contamination onto the die surface. Its extent depends on the material, the cure profile and the surface energy of the substrate.

Outgassing is the related problem in a sealed package. Volatiles released during cure or during subsequent reflow can condense on the die or the bond wires, and in a moulded package they can create pressure that separates the mould compound from the die surface. Choosing a material with low volatile content and curing it fully before encapsulation reduces both risks.

Inspection Methods

X-ray is the standard tool for finding voids in a soldered or sintered attach, because the density difference makes them visible. It gives a two dimensional projection, so a large void near the surface can hide smaller ones beneath it, and the resolution needed for a thin layer is demanding. Scanning from an angle helps confirm the depth.

Scanning acoustic microscopy is used where a polymer attach must be examined for delamination rather than voids, because ultrasound reflects from the interface between materials. The two methods answer different questions, and the comparison between them is the same one drawn in this guide to X-ray and AOI inspection.

Qualification and Reliability Testing

A qualified attach process is one that has been tested to the failure modes it will face: thermal cycling to fatigue the bond, high temperature storage to grow intermetallics, and moisture exposure followed by reflow to expose absorbed volatiles. Each test targets a specific mechanism, and passing one says nothing about the others.

Acceptance criteria should be written in terms of measurable quantities, such as a maximum void percentage, a bond line range and a shear strength minimum, and they should be checked on a sample from every lot. Reliable results depend on tight control of dispensing and placement, since the attach layer is formed by a process rather than by a single component. That is the same component tolerance and reliability principle applied to the package rather than to the board.

FAQ

What void level is acceptable in a die attach layer? It depends on the power density and the application, but many requirements cap total voiding at a few percent of the die area with no single void above a defined size in the central region. High power devices with the junction spread across the die need tighter limits than small signal parts.

Why does the attach pattern affect voiding? Because the pattern determines where air can go as the die descends. A pattern that leaves a continuous escape path lets air leave ahead of the flowing material, while a closed pattern traps a pocket in the centre. The correct pattern is a function of the die size and the material viscosity.

Is sintering always better than epoxy attach? It gives a much higher thermal conductivity and a bond that survives far higher temperatures, but it needs pressure, a longer process and a metallised die backside. For low power devices the process cost is usually not justified, and a filled epoxy is the better engineering choice.