Materials World Widespread use of BGA and CSP devices in combination with their finer pitches has necessitated additional package protection to ensure long-term reliability and good in-field performance. This, of course, is what the latest underfill technology has been designed to do – deliver required stress reduction and thus improve device reliability. Although first-generation underfills were engineered to accommodate for coefficient of thermal expansion (CTE) differences between the substrate and device, technology advancement and new packages have yielded additional uses for modern underfill systems. Specifically, handheld and mobile products that are subjected to higher levels of stress from drop, shock and vibration, benefit greatly from underfill support.

ImageBut even handheld and mobile devices have different underfill technology requirements depending on the application. For example, if the BGA or CSP is used inside a handheld product such as a mobile phone, PDA or portable music player, a capillary flow underfill is the best solution. Because these devices have a high probability of being dropped (and often), having underfill material evenly dispersed underneath the package and between all solder-to-pad connections is essential for high reliability. Other applications, though, are more conducive to less time- and cost-intensive underfill technologies such as cornerbond and edgebond materials systems. As discussed in previous columns, array devices within certain mobile applications such as laptops and gaming consoles that may experience stress during card assembly or shipping may not need as much support as those housed within handheld devices. Because the stress induced is generally much less and limited versus that occurring with handheld products, other underfill technologies can deliver more cost-effective protection for mobile applications.

The two most prevalent underfill technologies employed for mobile devices are cornerbond and edgebond. It warrants mentioning, though, that while manufacturers often use these terms interchangeably, they are two distinct products and are applied at different stages in the assembly process. This is an important to understand so that during material selection, you are certain that the proper material is used at the correct point in the process. Cornerbond technology is used inline, applied to the four corners or four edges of the pad and is cured during normal solder reflow. In Pb-free manufacturing especially, it is imperative that the material be compatible with higher temperature processes and that it permits component self-centering during reflow. And, while cornerbond is an inline process, it does require precise dispensing equipment.

Although cornerbond is one of the most cost-effective, high reliability underfill systems available for mobile devices, there is another alternative for manufacturers who want to apply material post-assembly and desire a less capital equipment-intensive solution: edgebond. With edgebond, manufacturers can dispense underfill post-assembly and cure the material using a UV or thermal process. These materials can also be applied manually, albeit with less precision than automated dispensers. Once deposited around the edges of the device, edgebond is cured using one of the two methods: UV or thermal. If UV cured, it is essential to use materials specifically designed for this purpose and this process. Edgebond materials must be of a viscosity such that the material does not flow far underneath the component, but remains at the edge so that sufficient light (or heat in the case of thermal cure) reaches all of the material. If a product that was not designed as a true edgebond is used, often the underfill flows well underneath the component and the material never fully cures – which can result in device failure. A fully cured material is imperative to the long-term reliability of the device.

Today’s underfill technologies are certainly not one-size-fits-all solutions. There are a variety of different systems for varying applications and it is important to understand each before selecting a material.

Renzhe Zhao, Ph.D., is technical manager of applications engineering at Henkel’s Electronics Group (henkel.com); renzhe.zhao@us.henkel.com.
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