Epoxy flux bested conventional tacky fluxes in drop testing, and are easily deposited.

Although fluxing – or no-flow—underfills have been on the market for some time, the drawbacks of these older generation materials have prevented their widespread adoption. Challenges with performance and reliability often negate the inline processing advantages of these materials. In a normal no-flow operation, underfill is applied to the substrate prior to component placement, and when the assembly passes through reflow, the material cures. But no-flows are susceptible to moisture outgassging from the substrate, which results in voids. To alleviate this issue, manufacturers often select reflow-curable alternatives such as cornerbond and post-reflow cured edgebond materials that do not fully underfill the device.

Corner and edge support underfills are an effective choice for certain devices, but there are numerous applications for which they are not well-suited. Therefore, to offer the protection of a full underfill with the inline efficiency of a reflow cure, new epoxy flux materials have been developed. These materials deliver a reflow curable formulation that offers both flux and underfill in a single product. The flux component of the material enables solder joint formation and the epoxy system offers underfill-like protection for each individual bump. Originally designed for underfilling large format CSP and BGA devices where flow rates and cure times of traditional capillary underfills can limit throughput, epoxy fluxes are also proving to be an effective material for emerging package-on-package (PoP) configurations as well as for ball attach processes, among others. The epoxy flux can be applied via screen printing, dipping, dispensing or jetting.

For PoP assembly, epoxy flux is a promising solution to one of the biggest challenges with PoP devices. While the level-one package assembly follows fairly routine surface-mount procedures, the assembly and subsequent long-term reliability of the level-two package is not as simple. Currently, the most common method used is to dip the bottom side spheres of the top package into a tacky flux, place the package and then reflow it. This permits solder joint formation but, unfortunately, the joints then remain unprotected and are subject to stresses from shock, drop and vibration. Alternatively, when epoxy flux is employed, the second level package is assembled using the same procedure but receives additional support from the adhesive. The level-two device is dipped into epoxy flux prior to component placement. Once the component is placed, the assembly moves through reflow where the fluxing action of the material enables robust solder joint formation and the epoxy component encapsulates each sphere and then cures. In addition to the added protection afforded by this new material, the in-line processing benefits and cost savings are substantial. Units per hour (UPH) is improved dramatically as subsequent dispense and cure steps are not necessary and costs are reduced through yield improvements and the elimination of dedicated dispensing equipment. In recent testing against other conventional tacky fluxes, epoxy flux offered the best drop test performance, as it was able to withstand the most number of drops before the first failure. This suggests that epoxy flux offers more protection than flux alone.

Similar results were revealed when epoxy flux was evaluated as a ball attach flux. Again, the reliability and strength of the material proved to be more robust than that of water-wash or no-clean tacky flux alternatives. When the shear strength of three solder sphere alloys was tested against four different fluxes (two water-washable fluxes, one no-clean flux and epoxy flux), the epoxy material emerged as the clear winner (Figures 1, 2 and 3). With each solder alloy, epoxy flux provided the strongest solder joint.


Dr. Renzhe Zhao is technical manager, applications engineering at the electronics group of Henkel (henkel.com); renzhe.zhao@us.henkel.com.