Fast Flow Capillary Underfills for High Throughput Assemblies

The continued trend towards manufacturing chip scale packages (CSP’s) with more demanding assembly processes for higher throughput and with lower yield loss requires new underfill materials to meet these demands. Many CSP’s currently in production that require underfill are typically underfilled at >80°C in order to obtain adequate flow times. Underfills designed with fast flow times at ambient temperatures and shorter cure schedules are critical to maximizing throughput. Also, lower cure temperatures enable greater flexibility in the manufacturing assembly process, allowing the placement of underfill curing steps upstream of other high temperature sensitive processes.

These industry trends have generated a need for capillary underfills that can be applied at temperatures close to ambient while still maintaining fast flow times. In this paper we investigated two approaches, epoxy and acrylic chemistries, for achieving underfills with fast flow times at ambient temperatures. Epoxy formulations were characterized with varying filler loading and filler particle size in order to balance the desired viscosity and enthalpy during cure. We also varied polymer composition and free radical generator type in acrylic formulations in order to obtain target properties such as flow time, adhesion, cure temperature, cure time, Tg, reworkability, and shelf life. We will discuss the characterization results, strengths and weaknesses of both the epoxy and acrylic approaches.