Addressing The ABF Substrate Shortage With In-Line Monitoring

Identify open defects earlier in the manufacturing process to limit the impact on yield.


Ajinomoto build-up film (ABF) substrate has been a key component in chip manufacturing since its introduction shortly before the turn of the millennium. Substrates made with Ajinomoto build-up film – an electrical insulator designed for complex circuits – are found in PCs, routers, base stations, and servers.

Looking ahead, the ABF substrate market will continue to grow, with revenue up last year due to the strong demand for 5G, high-performance computing (HPC), servers and graphic processing units (GPU), as well as from the automotive industry. According to Goldman Sachs, the total demand for the ABF market should maintain a CAGR of 28% from 2022 to 2025. Like so many other essential components in the global supply chain, there is a shortage of ABF substrates.

But rising demand and supply chain issues aren’t the only factors contributing to the shortage of ABF substrates. Larger package sizes and an increasing number of layers for these high-technology products also play a part; after all, these larger packaging sizes result in fewer packages per ABF substrate. And since the manufacturing of ABF substrate is a build-up layer process, a defect in any one layer can hamper the final yield of the entire substrate. Given these factors facing the ABF substrate market, yield control becomes even more important than it was before.

Defect repair

Yield control is a big-scope project, one that must take into account all the steps in the ABF process (figure 1). Traditionally, ABF and PCB substrate suppliers have relied upon the ability to repair RDL defects to preserve or increase yield. But with high-performance chips leading to smaller RDL sizes, repairing RDL defects has become more challenging. This is especially true for RDL open defects.

Fig. 1: Particle contamination can occur during any time in ABF substrate manufacturing process.

Defect inspection and metrology systems are critical tools in identifying open defects and determining where they come from. For a long time, ABF and PCB substrate suppliers have used inspection tools to identify open defects during after-etching inspection (AEI) and then repaired them. This is not an ideal solution anymore. High-performance computing requires high-density RDL, which in turn means that the number of open defects needing to be repaired increases, and with it the number of tools an ABF substrate manufacturing needs for repairing. This not only decreases yield, it drives up costs. In addition, as RDLs shrink, repairing open defects may no longer be an option since some layers are rendered unrepairable.

Faced with these challenges, in-line monitoring appears to be an effective tool at improving yield for high-end ABF substrates.

In-line process monitoring

From the first step of preparing the copper clad laminate (CCL) core boards to the last step of etching, particle contamination can occur during any time in the ABF substrate manufacturing process. If a particle comes in contact with the substrate during dry film lamination, it may cause an RDL open defect in the Cu plating (figure 2). Since this defect won’t be detected until several steps later during AEI, conducting in-line monitoring inspection can help identify an abnormal process problem earlier in the process. Inspecting ABF substrates using a defect inspection and metrology tool capable of in-line process monitoring after laser drill, baking and lamination can help users uncover the step during the manufacturing process in which a killer defect occurs.

Fig. 2: If particle contamination occurs under the ABF or dry film, it can result in an RDL open defect.

More packages per unit

The ABF substrate market will continue to grow over the next several years due to the strong demand of the high-end market driven by HPC, AI, and 5G. ABF substrate suppliers looking to capitalize on this demand by expanding their capacity can increase yield and deliver more packages per unit by using tools capable of in-process monitoring. Using such a defect inspection and metrology tool allows manufacturers to identify open defects earlier in the process and correct them, limiting the potential impacts of these defects on yields and the bottom line.


Eric says:

As the shrinking of RDL size, the pad pitch will be smaller than 50um. Does the traditional prober card test still can meet the requirement of open/short test? if not, how to do that?

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