Three Seconds of Silence When the CPK Report Came Out That Day
I remember a while ago, when we introduced a new product using advanced packaging, the production line ran for two weeks, but the yield was consistently stuck at an awkward, mediocre number. During the meeting, the R&D, PE, and PM teams all had grim expressions. When the quality department displayed the CPK report, the number 1.08 strikingly appeared on the screen, and the entire room fell into three seconds of silence. Honestly, everyone knew deep down that this number was unacceptable for shipment, but where exactly was the problem? It was then that I truly realized the quality challenges of advanced packaging were an entirely different level of game.
Where the Problem Lies
To put it simply, the biggest headache with advanced packaging is the forced "adhesion" of a multitude of components that originally operated independently and were considered "good" in quality. Previously, when we cut a wafer, each die was a single unit; at most, if it failed testing, it was discarded. Now? Imagine playing with LEGOs: previously, you only assembled one block; now, you need to seamlessly glue ten different-shaped blocks together with nano-level adhesive, without any skewing or air bubbles. This increases the number of interfaces and the heterogeneity of materials; if any single step goes wrong, the entire IC is scrapped.
Therefore, the key point is that advanced packaging's quality challenges are no longer single-process yield issues, but rather "systemic" integration problems. You cannot just look at the CPK of each independent step; you must consider their performance "when combined."
How It's Done in Practice
Frankly, we later put a lot of effort into the "interfaces." Previously, when we inspected wafer DPMO (Defects Per Million Opportunities), we might see 6210 ppm and consider it acceptable. However, in advanced packaging, if you have a stack with three layers of chips, defects can potentially arise at each interface of every layer. Assuming each interface has a defect rate of 6210 ppm, then for a three-layer stack, the defects caused by interfaces are not a simple summation. At this point, we treat each interface as an independent "opportunity point" for calculation, and even introduce stricter PPM specifications, for instance, reducing from the original 6210 ppm to below 300 ppm, otherwise, stacking would lead to disaster.
In other words, you need to approach this from three aspects: "material compatibility," "process parameter stability," and "inspection capability."
- Material Compatibility: Ensuring that differences in thermal expansion coefficients, mechanical strength, etc., between different materials do not generate stress during manufacturing or long-term use.
- Process Parameter Stability: Parameters like the adhesive thickness and curing profile for Die Attach, or the uniformity of line width and spacing in RDL (Redistribution Layer) processes, must be brought to an extremely high level of stability.
- Inspection Capability: You need to be able to catch tiny defects that were previously undetectable, making non-destructive inspection methods like X-ray and SAAM (Scanning Acoustic Microscopy) extremely important.
Common Pitfalls
In our CPK 1.08 case, the most common pitfall was the "out of sight, out of mind" mentality. Initially, we only focused on the yield of each individual process, believing that every step was fine. However, when the chips were stacked, packaged, and subjected to final testing, there were numerous open circuits, short circuits, and even failures in reliability tests. It was later discovered that tiny air bubbles at a certain interface, which were undetectable individually, expanded or ruptured due to the high temperature and pressure of subsequent processes, leading to failure. This is like checking every LEGO brick and finding it perfect, but only after assembly realizing the entire structure is shaky due to poor interface design.
Therefore, truly, don't assume that high yield in individual processes means everything is fine; the devils of advanced packaging are hidden in the interfaces.
One Thing You Can Do Today
Transform all interface defects from "acceptable" to "zero tolerance."