High-Temperature Reliability of Wire Bonds on Thick Film
Authors: Shen Zhenzhen , Reiderman Aleksey 
Published: November 2017

In a harsh environment, wire-bonded interconnects are critical for overall reliability of microelectronic assemblies. Aluminum is the dominating metallization of the die wire bonding pads and aluminum wires are used to achieve monometallic bonding system on die side. On the substrate side, a monometallic connection is not readily available and typically involves expensive aluminum thin-film deposition or labor-intensive bonding tabs. Nickel-palladium-gold galvanic or electro-less plating stacks are also used to improve bondability and reliability of non-monometallic Al wire bonds on the substrate side. However, these plating stacks do not perform well after excursions above 330°C that are needed for the attachment of die and passives prior to wire bonding. At these temperatures, both palladium and nickel diffuse through the gold and form surface oxides that degrade wire bondability. In monometallic wire-bonding schemes, in addition to aluminum wires gold wires within same assembly are often also needed, for example, when some die is only available with gold-plated bond pads, or to connect substrates to gold-plated pins of hybrid housings. A universal substrate metallization, compatible with aluminum wire and gold wire, is therefore desirable. Thin-film substrates produced by sequential deposition and etching of gold metal, barrier metals, then aluminum metal is a good working solution, but it can be as much as ten times more expensive than other types of substrates. Printed thick-film metallization, a well-established technology, have been widely used for hybrid substrates. Silver-based thick films are inexpensive and capable of accepting aluminum and gold bonds. However, the silver-aluminum bonds are seldom used because of intermetallic formation and subsequent degradation triggered by multiple factors like temperature, humidity, and the presence of halogens. Pd and Pt are often added to the Ag thick films to decrease this effect, but potential usability and the reliability of these formulations in extreme temperature environments is not well researched.

For this study, samples of Pt/Ag thick-film metallization were printed on Al2O3 substrates, and 25-um and 250-um aluminum wires and 50-um gold wires were wedge bonded in daisy chain to the substrate. The test vehicles were subjected to high-temperature testing at 260°C and 280°C. Thermal cycling tests from −20°C to 280°C were also performed. Mechanical and electrical characterizations of the wire bonds were conducted periodically. These tests included resistance and pull-strength measurements. Failure analysis of the bond joints was performed to understand the results of the tests.

The 250-um Al wire and 25-um Al wire showed no significant changes until a critical time-at-temperature was reached. After reaching this temperature, the wire/substrate interface resistance rapidly increased to values as high as 40 Ohms for the 25-um Al wires. However, the pull strength remained within standard throughout the tests of up to 1200 hours. The relationship between time to failure and the temperature is presented in the paper. There was a four times life increase of bonds with every 20°C. With gold wires, no dramatic increase of bond resistance was observed, only a slight increase with time. The pull strength of Au wires remained stable throughout the time at high temperature. The tested Ag/Pt thick film metallization was found to be compatible with bonding of the gold wires and the aluminum wires for high-temperature applications up to an Arrhenius equivalent of 800 hours at 260°C. Additionally, Parylene HT coating was vapor-deposited on one set of 250-um Al wire-bonding samples. This set of samples demonstrated doubling of its useful life as compared to the uncoated samples.