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Evaluating cost reductions without impacting long-term reliability is an exercise assembly specialists undertake regularly. “What if we used a different material? What if we sourced components from a different vendor? What if we altered this part of the process?” The fact is that trimming even pennies per assembly can have a very large impact on manufacturer profitability and, in the case of EMS firms, may be the difference between winning or losing business. Of course, cutting costs without cutting quality is often a very fine line to walk.

When analyzing costs from a materials perspective, there are several factors than can impact the cost of a material to an assembler. Certainly, the components of the material are top of the list, but equally important is the method by which the materials are applied. As the adage goes, time is money. So, if cycle time and throughput can be improved say, for example, by eliminating a step from the process, then that factors greatly in the overall final cost of the product. Once such materials-focused cost-saving approach that is increasingly being employed by assembly firms worldwide is replacing traditional, capillary underfill processes with an alternative, corner dot – or cornerbond – underfill material.

Cornerbonding is not intended to be used with CSPs that will reside within a handheld device such as a cell phone or portable music player that has the very high likelihood of being dropped. For those applications, a capillary flow underfill is still the best option for long-term reliability. But, for CSP devices that will be incorporated into other types of mobile devices such as laptops or into desktop applications like PCs and gaming consoles where vibration during shipping may be problematic, cornerbonding is the optimum solution for reliability and cost-effectiveness. 

With cornerbond technology, dots of underfill material are dispensed at the four corners of the CSP pad site prior to component placement (Figure 1). One of the big advantages to this approach is that it can be incorporated into a standard manufacturing process, utilizing existing equipment with curing taking place during normal solder reflow. And, with the newer generation of cornerbond products, the materials permits self-centering during reflow to accommodate for slight misalignment during placement, which in turn improves long-term reliability and improves yield. Unlike capillary underfills, cornerbond requires no dispensing process after reflow and no secondary curing step, which saves on equipment expense and helps maximize the all-important UPH (units per hour). Plus, some of these materials are reworkable, permitting manufacturers to avoid scrapping an entire assembly if there is only one defective device. All these benefits –capital equipment expenditure reductions and significant cycle time improvements that can be realized – combine to provide significant and measurable overall manufacturing cost savings.

Figure 1. Cornerbond materials dispensed at the four corners of a CSP provide a cost-effective solution to capillary underfill for some applications.

As with any process, there are important points that should be considered to ensure the most reliable and error-free result when cornerbonding. Some four-corner underfill materials are UV curable and should be employed with caution to safeguard against other forms of reliability issues. When material is located underneath a component, adequate UV light may not reach the material and uncured epoxy and, thus, compromised reliability may be the result. Also, when using pre-reflow dispensing of cornerbond materials, careful pad design and underfill placement is essential and, when applied properly, this technology is ideally suited for CSPs and BGAs. 

Overall, the cornerbond underfill solution provides excellent mechanical support for CSP devices being used in certain applications where moderate stress will be induced. It is cost-effective, simple and can be used inline. For manufacturers seeking to reduce cost and maintain a high level of reliability for CSP-containing products, cornerbond technology is the ideal solution.

Renzhe Zhao, Ph.D., is a senior applications engineer at Henkel’s Electronics Group.