Epoxy Flux Technology - Tacky Flux with Value Added Benefits
This article is based on an original publication by Loctite.
Introduced more than 30 years ago, underfills have supported many modern advancements which would not have otherwise been possible. Sustained developments - such as filler technology, increased control of flow rates, new cure mechanisms, improved modular properties, and alternative application techniques - continue to introduce enhanced capabilities to the industry. However, as the market maintains its trend toward increasingly flexible and reliable miniaturized devices and components, underfill system capabilities will require continued enhancements.
Currently, the most common types of underfills used are capillary flow materials, fluxing (noflow) underfills, cornerbond and edgebond systems. While each is viable for certain applications, newer components will benefit from an inventive underfill material technology - called epoxy flux - in the reflow cured encapsulate class. This new material system is enhancing applications from semiconductor packaging to PCB assembly.
Epoxy Flux Underfill Technology
Developed for process efficiency, this material system provides a flux component to enhance solder joint formulation in conjunction with an epoxy system that protects components and devices by encapsulating individual bumps. Cured during the reflow process itself, epoxy fluxes provide an in-line alternative to other underfill mechanisms to eradicate the requirement for a dedicated dispensing system and its associated time requirements.
Epoxy fluxes additionally offer deposition flexibility, and can be dipped, jetted, dispensed or screen printed, depending on application and process. While in-line processing is not unique to epoxy flux, no other underfill materials deliver an equivalent level of in-line processing. For instance, using the noflow technique, material is cured during reflow, which can create voids caused by moisture outgassing from substrates. Because epoxy fluxes only encapsulate individual bumps and spheres, they leave channels which allow volatile gasses to escape while maintaining solder joint protection.
While there are a range of tacky flux formulations used for a variety of applications, epoxy flux may prove the more effective from a reliability standpoint. Studies show that epoxy flux material delivers the strongest solder joint when compared to traditional flux formulations.
Use of epoxy flux is also proving beneficial for emerging package-on-package, or PoP, configurations, and is particularly useful for warping issues in the second-level assembly which can lead to stretched or broken solder joints. When using epoxy flux, the solder joint is formed during reflow and each individual sphere is encapsulated with epoxy for an added level of protection.
With their large footprint devices, BGA and CSP assembly operations benefit from cost efficiencies delivered by epoxy flux. These devices require a higher volume of standard underfill to cover the device area entirely, and flow rates and cure times of these large volumes may impact throughput rates. On the other hand, epoxy flux allows for in-line processing, eliminating dispensing equipment, cure ovens and extra processing time.
Traditional underfill systems are struggling to meet the demands of modern package configurations, finer pitches and the need for higher throughput rates. While standard underfills will always have their uses, for many other emerging technologies, older material systems cannot deliver in-line processing and high reliability advantages. Epoxy flux materials offer all this and more, with added versatility and a broad application range for both packaging and board assembly environments with unprecedented flexibility.