Die attach materials impacts to copper wire bonding: New challenges

Abstract

Copper wire bond remains a challenge especially on bond pads with sensitive pad metallization. Extensive work and analysis are needed at the onset of the packaging development phase to meet the right level of manufacturability and reliability requirement. Wire bond related issue is not uncommon even after qualification and after years in high volume manufacturing. The process development gets even tougher for finer wire diameter, bond pads with less than 1μm aluminum thickness and of higher via density structure. A workable wire bond process window is needed upfront. Key is to have a window defined to balance between pad cratering and lifted ball bond due to the harder property of the copper wire. Wire bond process optimization commonly focus around bond time, bond force and bond power (USG). Other factors generally evaluated for copper wire bonding include incoming bond pad cleanliness, bonding pad surface oxidation, wire oxidation during EFO and forming gas flow rate. A lesser known variable, die attach, is often overlooked in packaging or gold to copper wire conversion. Die attach material out gassing at cure is a common hypothesis whenever there is bond pad surface contamination issue. But with the advancement in technology and new material development, out gassing risk is low if the material is cured per the supplier recommendation. Therefore, investigation on weak bond is generally focused more on the wire bond process and incoming bonding pad condition rather than the die attach material. In this paper, investigation on non stick and copper wire bond was carried out. A well defined copper wire bond process on sensitive bonding pads is developed using statistical approach in the form of design of experiment (DOE) and response surface methodology (RSM). Surface analysis methodology using SEM, EDS and AES were also utilized to check for bond pad contamination. To understand deeper into the issue and mechanics of the failure, process mapping and brainstorming sessions have also been carried out. Variables involving wafer saw, pre-wire bond plasma, different batches of wafers, die attach bond line thickness and tilt variation have all been studied thoroughly. Eventually, the root cause of copper wire bond issue been identified to be die attach material related. A strong correlation has been found between lifted wire and the die attach material modulus. There is no more lifted ball concern after changing of the die attach material to a material of a suitable modulus. There is no significant difference in the die attach bond line thickness, coverage and voids among others, comparing with the original die attach material. All the wire bond responses including pad cratering, Al-Cu intermetallic (IMC), wire pull and ball shear have been validated to have no issue at a larger sample size. Reliability assessment in the form of preconditioning, temperature cycling and bias HAST have also passed with no sign of interfacial mechanical failure or test failure. The modulus of elasticity of the die attach material is important and must be a key consideration in copper wire qualification. The elasticity of the material changes from room temperature to copper wire bonding temperature. A low modulus at the bonding temperature can cause wire bondability problem such as lifted ball bond due to ultrasonic power loss during ball bonding. Therefore, selection of a die attach material with a suitable range of modulus of elasticity is critical in the integrity of copper wire bonding.