The Rise Of Copper Wires In Automotive ICs

In 2011, the price of Gold (Au) surged to $1900/oz which had a drastic impact on Wirebonded ICs using Au wires. IC suppliers scrambled to convert from Au to copper (Cu) wire on as many products as they could. However, automotive ICs were reluctant to make the jump due to lack of reliability data and performance track-record. However, today’s automotive ICs are big users of Cu wires driven by cost and reliability considerations.

In 2021, the automotive semi market was $52.6B and it has grown 5x since 2001 (Source: Omdia). Wirebond ICs are the dominant part of this growing market today. Wirebond packaging represents more than 90% of the automotive packaging market (Source: Yole).

Source: Yole Developpement Automotive Packaging Markets and Technology Trends Report 2019

Old automotive ICs continue to use Au wire, but new wirebond ICs are almost always designed with Cu wires. Other than high cost, Au wires also show reliability issues after High Temp Storage Life Testing (HTSL). Specifically, Kirkendall voids are observed in the joint between the wire and the Aluminum pad. Cu wires do not show such voids as seen in the following figure.

HTSL 1000h @ 175C Results

For new applications, Cu wires have become the default due to lower cost, better conductivity, HTSL performance, and now mature equipment/process capabilities. However, Cu wires do pose some challenges:

1. Cu wire is harder than Au wire and the die bond pad structure must be evaluated for compatibility with the higher stress associated with Cu wirebonds. The presence of active circuitry under the bond pads can be at risk for use with Cu wires. This is one of the main reasons that many older devices using Au wires couldn’t be retrofitted with Cu wires. Even with compatible devices, material and process controls are necessary to protect against pad damage.
2. Although Cu wire shows relatively better HTSL & Temperature Cycling (TC) performance, it is more susceptible to HAST (Highly Accelerated Stress Test) reliability failures. The presence of Chlorine (Cl) ions in the Epoxy Mold Compound (EMC) leads to Intermetallic Compound (IMC) corrosion that can cause an open joint failure. Other aspects of the EMC such as PH level and Sulfur content must be also considered.
3. Cleaner assembly environment is important to eliminate contaminants that can lead to reliability failures with Cu wires. This requires tighter controls and environment monitoring to reduce contaminant particle counts and their possible sources.
4. Other BOM components such as leadframe/substrates, die attach and the specific variation of Cu wire chosen are important. There are different types of Cu wires.

a.  One of the most popular Cu wires is the AuPCC (Gold and Palladium Coated Copper Wire). This wire has a 4N Cu (4 “Nines” = 99.99% purity) core with a Pd coating and Au coating.
b.  Two other variations are also being used for higher reliability:

i.   Alloyed Cu wire uses 2N Cu (99% purity) doped with other materials. The goal is to achieve improved High Temp Storage Life performance.
ii.  Hybrid Cu wire is similar to Au PCC wire but uses a 2N Cu core doped with other materials. This is intended to achieve superior performance in automotive reliability testing. The Hybrid Cu wire also has better workability compared with Alloyed Cu.

The Automotive Electronics Council (AEC) has released a reliability spec called AEC-Q006 that must be used with AEC-Q100/101 to qualify automotive ICs using Cu wires. This spec outlines stringent reliability criteria to ensure that Cu wire ICs can perform reliably in an automotive environment. To achieve this reliability, careful material selection and process optimization are necessary. Cu wire is now the wire of choice in all wirebond applications and its use will only grow in automotive applications in the future.