On July 1st, Toshiba Corporation's Semiconductor Company and Storage Products Company consolidated to form Semiconductor & Storage Products Company.This page describes reliability information of semiconductor products.

Failure Mechanisms

[As of April, 2011]

Assembly Techniques

Chip Mounting (Die Bonding)

The chip mounting method used is generally the conductive paste method for lead frame type packages and the solder method for certain power products. The particular failure mechanisms of these two methods are described below.

Conductive Paste
This method uses resin adhesive to bond the chip to the substrate. The resin adhesive usually consists of epoxy or polyimide, and is generally prepared by adding flaky Ag powder into the resin to provide electrical and thermal conductivity.
A problem with resin adhesive is the gas generated by the materials, which can cause a failure due to corrosion, migration, surface leakage or V_th shift. Most of the gas is moisture, with some components of amine or halogenide.^{Note 1}
For this reason, careful consideration should be given to the adhesive agent used. The agent should have minimal impurities.
Solder
Solder chip mounting is widely used for power transistors. However, thermal cyclic stress is generated through heating and natural cooling caused by repeatedly switching the transistor on and off, which accelerates the degradation of the solder bond. This is referred to as thermal fatigue. The degradation of solder can subsequently increase thermal resistance, resulting in device breakdown due to local heat generation.
To improve reliability with respect to thermal fatigue, it is critical to select the proper solder material and control the environment of the solder bonding work area.

Bonding

There are two bonding methods used to connect the leads to the semiconductor chip electrode area: thermal compression and ultrasonic.
The following describes the bonding process and the failure mechanisms related to bonding wire.

Au-Al Alloying
When bonding Au wire to Al or Al wire to Au film and subjecting the bonding area to high temperature, a formation of purple alloy (AuAl₂) is often observed. This alloy is referred to as "purple plague." In contrast, Au₂Al which readily occurs when the proportion of Au is high, has higher electrical resistance and is mechanically weaker than AuAl₂ and is referred to as the "white plague."
Since Au and Al have different diffusion constants, voids accumulate depending on the change in volume of the generated compound, forming nests (cracks) where there is a high concentration of Al, resulting in bonding strength degradation and increased resistance. Bonding degradation will lead to ball peeling, temporary ball peeling during operation at high temperatures or opens due to the stress that occurs from vibration and the difference in molded resin and wire thermal expansion coefficients. However, such degradation can be considered virtually unproblematic if the heating process is properly controlled during the manufacturing process.
Mechanical Stress
With resin-encapsulated devices, thermal cyclic stress can cause additional mechanical stress on wires due to the difference between the thermal expansion coefficients of the resin and wire, resulting in an open wire due to wire fatigue. Also, wire deformation during molding can cause wires to come in contact with each other or with the edge of the chip under high temperatures, resulting in an electrical short. Countermeasures for this include process optimization and automation.
In hermetically sealed devices, wires formed in a loop shape in the interior can open due to shock or vibration. This should be taken into account, especially when the device is subjected to ultrasonic cleaning.
Furthermore, mechanical damage during bonding can, in some cases, cause secondary device failure. However, this can be counteracted by optimization and/or automation of the manufacturing process.

External Plating

The lead electrode plating process is performed to improve solder wettability during mounting and prevent corrosion. In general, there are two treatment methods: electroplating and hot dipping.
For electroplating, Sn-Pb (tin-lead) plating and Pd (palladium) plating are widely used. Recently, however, Sn-based solder plating such as Sn-Ag (tin-silver), Sn-Bi (tin-bismuth) and Sn-Cu (tin-copper) plating that does not contain lead, and Sn (tin) plating that suppresses whisker growth are increasingly applied, reflecting a switch from Sn-Pb plating to lead-free plating. For hot dipping, Sn-Pb plating is used. However, with the development of lead-free products, a switch is being made to Sn-Ag hot plating and electroplating. The following describes the representative failures associated with plating.

Tin Whiskers
Although advances in the development of an Sn plating that suppresses whisker growth have been put to practical use in recent years, one of the reasons the external plating, which was originally Sn plating, has been switched to Sn-Pb plating is whisker growth. Although the addition of lead suppressed whisker growth, with the increase in Sn content in external plating material due to the switch to lead-free plating, caution with regard to whisker growth is required. The Sn content in Sn-Pb plating is 63 to 90 wt% in comparison to 95% or higher in lead-free plating. An Sn whisker is generally described as an Sn protrusion caused by oxidation, diffusion and compression stress that mechanically occurs. The Sn whisker can be found in the shape of a needle, nodule or spiral. The needle-shaped whisker in particular can grow quite long, resulting in electrical shorts between leads and, consequently, device failure.
Migration
Ion migration occurs with a variety of metals, but is especially known to occur with Ag. Migration is not viewed as largely problematic in Sn-Pb plating, but does require caution in lead-free plating that contains Ag. Migration refers to the movement of the metal component (plating) above the non-metal component (mounted substrate) in an electrical field. Because the metal that moves is conductive, migration causes an electrical short between leads, resulting in device failure.

Note 1: Bibliography. Fujitsu, Koike, Watanabe, Uno, Umaba; "Semiconductor Power Device Thermal Fatigue Analysis," 10^th JUSE Reliability and Maintainability Symposium Proceedings, (1979), p. 375