Suppression of Interfacial Delamination in High-Power Devices by Advanced AP Coating

In recent years, solders have been widely adopted as a die-attach material for power devices. However, in high-power devices, exposure to large currents and high-temperature environments has made the low adhesion at the interface between solder and epoxy mold compounds (EMCs) an issue. This poor adhesion can cause interface delamination with EMCs, potentially leading to decreased device reliability. In the automobile industry, where high reliability is required, poor adhesion becomes a critical issue. As a result, the demand for power devices that are resistant to temperature changes is increasing.

Delamination is likely to occur mainly at the interface between EMCs and solder. This is because the coefficient of thermal expansion of solder and EMCs differs significantly, and they are also not chemically bonded. Since the adhesion strengths between solder and EMCs are not strong enough to withstand all the delamination inducing stresses, the devices tend to have delamination. Methods such as surface roughening of lead frame (LF) and plasma cleaning have been used to mitigate the issue of delamination between the LF and EMC. However, measures to prevent delamination on the solder are difficult.

This study evaluates adhesion promoter (AP) coatings to achieve no interfacial delamination for power devices. The adhesion promoters can bond inorganic materials like dies, wires, solders, and lead frames to the organic EMCs. Therefore, efforts focused on silane (silicon-hydride) coupling agents. The silane coupling agents are molecules containing silicon-hydride (SiH) groups and another organic chemical functional group. To attach, the SiH groups form covalent bonds with the hydroxylated oxide layers on the inorganic materials under the presence of water, while the chemical functional groups cross link with the functional groups in the organic materials. This unique chemical structure enables covalent bonding with both inorganic and organic materials. Silane coupling agents fit as adhesion promoters to be coated on the power devices, which consist of a combination of inorganic and organic materials.

The AP coating starts by placing the die and wire bonded LF into a bath of AP diluted by a mixture of alcohol and deionized water. After that, the LF dries in clean room conditions at room temperature. As a result, this is a very simple and easy coating process. Evaluations were conducted on the AP coated power devices and uncoated samples by the stringent moisture sensitivity level (MSL) 1 (MSL JEDEC Level 1) condition and 1000 thermal cycle (TC) test. The delamination was evaluated using constant-depth mode scanning acoustic microscope (SAM) after assembling discrete packages. None of the coated units delaminated at die paddle in the initial state. After MSL 1 condition, delamination on the uncoated units starts on the solder and this delamination expands after 1000 TCs. After 1000 TCs, all samples in the uncoated group delaminated. However, none of the samples with the AP coating delaminated up to TC1000. These results indicate that the AP coating is effective as a measure against interface delamination.

This testing indicates that the interface reliability of high-power devices can be improved by an advanced AP coating under MSL 1 and TC 1000 cycles conditions. The AP coating has achieved zero delamination by increasing the adhesive strength between solder and EMC, which has been considered difficult. By using Silane coupling agents, adhesion strength can be expected to be enhanced, not only for solders but also more organic and inorganic materials. These results will make a significant contribution to the electronic system’s reliability for the automotive industry.