Epoxy Flux Material and Process for Enhancing Electrical Interconnections

Neil Poole, Ph.D., Elvira Vasquez, and Brian J. Toleno, Ph.D.
Henkel Electronic Materials, LLC
Irvine, CA, USA

There are two main drivers that are causing electronic device manufacturers to look into methods other than solder paste to form electrical interconnection on area array devices. The first driver is the move to smaller and smaller pitch. On hand-held devices it is very common to see 0.5mm and 0.4mm pitch CSPs, with 0.3mm pitch WLCSPs becoming more prevalent. While there are solder paste materials that can be used to print these fine features, these materials (and the accompanying stencil thickness) are not always suitable for the other components on the assembly. The other driver is the stacking of CSPs.

Package on package (PoP) process has been well studied and documented over the past few years [3 - 5], here once again, printing solder paste for the second level interconnect is not practical. In both cases there have been studies looking at tacky flux and/or dipping solder paste in order to form the solder joints [1,2, & 6]. Both of these methods can be used to produce good solder joints, but due to the end nature of these devices these solder joints typically need to be enhanced against drop and vibration by the use of an additional underfill process.

In this paper we outline the material and the processes used to produce devices with a novel epoxy based fluxing system. This material provides the wetting and activity to create a solder joint as well as providing some reliability enhancement due to the epoxy adhesive portion without the use of harmful solvents. In addition, we will also discuss the process capability of this material with respect to repeatability.

The pace of new package development is tremendous. Consumers continue to demand higher functioning, low cost products and manufacturers must keep pace. High volume, high reliability solutions are the only answer for optimization of production environments and new underfill materials technology is enabling these advances.

New package configurations, finer pitches and the need for ever increasing throughput rates are pushing current underfill systems to their limit. Of course, there will always be a place for traditional capillary underfills. But, for stacked packages, large footprint array devices and many other emerging technologies, older materials systems cannot offer the in-line processing advantages in tandem with the reliability required for these new products.

In this paper we have shown the process robustness of new dip processable next-generation epoxy flux systems. This has been demonstrated with multi package builds with 100% yield. These components and test vehicles were then shown to be more reliable with respect to drop and thermal cycling versus solder paste alone.

Next-generation epoxy flux materials, however are providing not only the UPH, performance and reliability required for high-volume manufacturing, but also offer a level of versatility heretofore unavailable. With a dual function flux and underfill in one material, epoxy fluxes have a broad application range for both packaging and board assembly environments. With capability for PoP assembly, large area array device assembly, protection and much more, manufacturing firms can conceivably source one material for production of various products.

In this paper we discuss the process window and capability of dipping, similar formulations have been used and applied via dispensing, screen printing, and jetting. This provides unprecedented manufacturing flexibility.