Breakthrough development of new die attach method with high conductive wafer back coating

The epoxy dispensing process is one of the most commonly used die attach methods in the semiconductors assembly process. Some major packages such as QFN package can only utilize epoxy die attach method due to the temperature constraint of the leadframe tape (withstand less than 300□C). However, the epoxy dispensing die attach method has encountered several major quality issues even after running production for long time. These issues could not be eliminated due to lack of possible effective permanent corrective actions. Such quality issues including application/field failures due to intermittent epoxy-shorting, epoxy overflow and insufficient epoxy. Some of the common root causes were nozzle clogged or deformed, pressure effect inside syringe changed with different content level, variations of epoxy material mechanical properties (viscosity, modulus,...), ... and etc. In order to eliminate all these quality issues, the conventional epoxy dispensing method requires some evolutional changes. From the comprehensive study on various die attach methods, and one of the best possible options is Wafer Back Coating (WBC). The WBC method requires epoxy paste printing on the wafer backside and then sawn together with the wafer. At the die attach machine, the singulated die (with epoxy coated underneath) would be bonded directly to the carrier leadframe. Therefore, the WBC method enabled the lowest dependency level of epoxy handling in the die attach process. Most of the epoxy dispensing major issues that dependent to human setup and judgement can be eliminated. Unfortunately, the high conductive WBC does not exist yet in the semiconductors industry, mainly because such material was considered impossible to be developed in production mode. The WBC material available is only for non-conductive and medium conductive applications. The main challenge of developing high conductive WBC material is due to its large weight content must be filled with Silver (Ag) to achieve high conductivity. Such high Ag filler content can cause severe quality issues in multiple assembly process steps. In the printing process, Ag filler may protrude out causing uneven surface. This caused poor adhesion to the mylar surface that is holding the wafer. During the high speed wafer sawing process, those poorly adhered die will fly-off, chip or even crack. Meanwhile, the Ag filler can also affect the die attach adhesion quality as it is blocking the function of resin to adhere to the leadframe and wafer surface. The adhesion strength can be very bad until die lifted in the wire bonding process. As for reliability performance, crack line was found propagating from leadframe surface, through WBC, then cracked at wafer backside surface. The failures occurred in both adhesive and cohesive strength. A joint efforts project was initiated with a major epoxy supplier in performing comprehensive WBC development study. In the initial stage, there were a lot of quality issues occurrences, sometimes ended up with zero assembly yield. Furthermore, some slight changes in formulation can cause strong confounding effect, resulted in getting unpredictable bad results. In the later stage, proper evaluations were designed and executed by following the Six Sigma DMAIC methodology. Many engineering samples were created in the screening stage, deriving each confounding effect to details independent effect. Some new interim tests were also created in order to evaluate the initial mechanical strength and electrical performance of any new WBC epoxy samples. Some of the breakthrough actions included optimization of catalyst types (accelerate cross linking), Ag filler types, Ag filler size mixture, Master Batch robustness, molecule chain length, hardener types (promote cross linking reaction), etc. Eventually, some of the new WBC engineering samples started to pass most of the assembly requirements. However, new issues occurred when the electrical performance failed inconsistently on some extremely sensitive devices. Further deep dive study showed that the WBC inconsistent performance was related to the variations found in both epoxy manufacturing process and assembly printing process. All the previously optimized process window needs to be further tighten. Some new controls were also introduced in both processes, then WBC material performance becoming more repeatable in production scale. This new die attach method with WBC is the ultimate solution to eliminate all the epoxy dispensing quality issues, from assembly process to reliability performance. Furthermore, the WBC method also provides other positive contributions such as throughput improvement (epoxy dispensing is the normal bottleneck process step), consistent electrical performance, competitive cost and utilization of maximum die size (zero clearance required between die edge and flag) for better functionality. In the near future, the new WBC die attach method will replace the entire epoxy dispensing process. This change will enable the opportunity to gain the most distinguished competitive advantages to run the most robust assembly processes in comparing to other semiconductors companies.