STEP 5: Advanced Underfill Technology

Estimated reading time: 8 minutes

By Brian Toleno, Ph.D., the electronics group of Henkel

Developed to compensate for the coefficient of thermal expansion (CTE) inconsistencies between the PCB and silicon devices, underfill technology has expanded to deliver shock, drop, and vibration protection for today’s advanced devices. Not one system is right for every application. This article explores various underfill technologies including traditional capillary flow underfills, reflow cured encapsulants, edgebond systems, and cornerbond materials.

So much has been written about underfill materials that some might argue it’s a topic not worthy of further discussion. Most electronics professionals – even those new to the industry – understand the basic purpose of an underfill: to provide mechanical support for the array package by filling the gap between the component and the board. These protective materials originally were engineered to offset the coefficient of thermal expansion (CTE) differences between the substrate and the device and, while this still is a primary underfill function, technology advances have necessitated the use of underfills for device protection from many other adverse conditions such as shock, drop, and vibration. Particularly in the lead-free age, the finer pitches of smaller-footprint packages combined with inherently brittle lead-free solder joints dictate the use of underfill technology for stress relief and long-term device protection. As package designers continue to develop highly miniaturized devices and the lines between board assembly and semiconductor package assembly become ever more blurred, underfill systems must keep pace to deliver the reliability required by the end user of modern devices. Each underfill type has relevance for a variety of applications, offering assembly specialists considerable manufacturing flexibility and several cost-effective options.

There is plenty to discuss about developments in underfill materials. In fact, today’s miniaturization trend sees PCB assembly firms demanding package-level underfill properties and performance for board-level products: a tall order to say the least. As requirements for higher throughput continue, it is clear that underfill materials with in-line, fast-cure capabilities will emerge as the most cost-effective products for certain applications. Not one underfill solution fits all and, while there are multiple formulations and applications, this article addresses four primary underfill systems: capillary flow underfills, reflow-cured encapsulants, cornerbond, and edgebond materials.

Capillary Flow Underfills

Without question, capillary flow underfills are the choice materials for advanced chipscale package (CSP) and BGA devices that require a high level of reliability, particularly those in handheld devices that will be exposed to higher-than-normal stress. For these devices, next-generation capillary underfills provide improved flexibility, fracture toughness, modulus, and adhesion. The latest formulations deliver more robust protection – especially for lead-free interconnects – than older underfill systems (Figure 1). Miniaturization and higher I/O counts of advanced CSP packages also require a higher level of protection to ensure greater reliability. This is achieved with capillary flow underfill materials.

Figure 1. Newer-generation underfills provide more lead-free joint protection than their older-generation counterparts and non-underfilled devices.

Traditional PCB assembly processes are migrating more toward package-level manufacturing techniques. As board-level area array devices get smaller, the underfills at the board level need to provide increased thermal cycling reliability. For example, 0.4-mm wafer-level CSPs (WLCSPs) are similar to the flip chip devices found inside early-generation QFP or BGA packages. In fact, even the underfilling process for WLCSPs mirrors that of underfill methods used for traditional flip chip applications. In both cases, dispensing equipment deposits underfill material along one or two edges of the device; material flows under the package and fills all the gaps between the CSP or flip chip and the substrate. While the processes are close, differences exist in the underfill systems used. Generally speaking, underfills used in package-level flip chip applications require JEDEC-level compliance and higher standards for moisture resistance than their board-level counterparts. However, as mentioned previously, the thermal cycling reliability standards are approaching each other for both applications. To meet the emerging underfill demands of assembly specialists, materials scientists now are investigating the feasibility of underfill formulations that address board-level requirements – ease of use, simplified storage, processability, reworkability – and deliver package-level thermal cycling reliability performance. Materials cannot yet deliver on these demands, but this is where the industry is driving. Development work is ongoing.

Reflow-cured Encapsulants

There are several types of reflow-cured encapsulants, the most well-known being no-flow underfill encapsulants. When deploying these types of materials, operators can perform application and cure in-line. Underfill is deposited onto the substrate prior to chip placement and, once the component is placed, the board travels to reflow and the underfill cures in a eutectic reflow cycle (Figure 2). There can be challenges with this process, including moisture outgassing and/or component floating, so many manufacturers have moved to alternative materials that are reflow-cured but not full underfills.

Figure 3: Reflow-cured underfill technology offers process efficiency for certain applications.

An emerging technology in this class of products is epoxy flux, whereby the underside component spheres are dipped into epoxy flux prior to placement. The device is placed and when the package travels through reflow, the flux provides the action necessary to form a solder joint and the epoxy encapsulates the solder spheres, delivering added support and protection. Because this method allows for both solder joint formation and sphere encapsulation, it offers greater process optimization. The equipment and time required for traditional capillary flow underfilling can be eliminated and throughput enhanced. Epoxy flux materials also show potential for package-on-package (PoP) processes. In a PoP process, one component is placed on top of another component during board assembly. The top of the level-1 component has pads along the perimeter to facilitate level-2 component attachment (Figures 4A and B). One of the biggest challenges facing PoP assemblers today is the attachment method used for the second-level package.1 Currently, many PoP device assemblers are using tacky flux dipping to attach the component. Like epoxy flux materials, the component is dipped in tacky flux prior to placement. When the assembly moves through reflow, the flux activity allows solder joint formation. However, tacky flux only facilitates solder joint formation; it does not provide added device protection. When an epoxy flux is used instead, the interconnect is formed and the solder spheres have an added layer of reinforcement from the epoxy encapsulation. More work remains to be done to fully understand the benefits of this process, but early data suggests that epoxy flux may deliver a two-tiered solution to the PoP placement and protection challenge.

In-line or Post-assembly Cost Effectiveness

Like reflow-cured encapsulants, cornerbond material properties allow for cure during the normal reflow process. However, unlike the epoxy fluxes mentioned above, these materials do not – and should not – interfere with the soldering process. The underfill is dispensed at the edges of the CSP pad site prior to component placement; the component is placed; and the device and board travel through the normal assembly process. This method of underfilling delivers several cost benefits including the ability to use existing, in-line dispensing and reflow equipment as well as improved units per hour (UPH) volumes achieved through a faster, in-process cure (Figure 3). When evaluating cornerbond materials, select a formulation that allows for device self-centering and is compatible with the higher temperatures of lead-free processing. Self-centering cornerbond materials compensate for slight placement misalignment, allowing the proper collapse of solder spheres, which improves interconnect reliability. Without this characteristic, cornerbond underfill materials may prohibit robust solder joint formation and adversely affect device reliability. Cornerbond materials are not suited for CSPs and BGAs used in today’s common handheld devices, subjected to multiple stresses. For those applications, capillary flow underfills are the best solution. Cornerbonds offer excellent process efficiency and device support for mobile products such as laptop and desktop computers, where vibration stress occurs only occasionally.

Figure 2. Basic process for no-flow underfill reflow-cured encapsulants.

For manufacturers that prefer to apply underfill post-assembly instead of in-line, edgebond underfills are an excellent alternative. Edgebond underfills are applied after board assembly, deposited around the edge of the device either manually or via an automated dispenser, which is the most accurate method. Once around the package, edgebond underfills are cured using either a UV or a thermal cure process. With a UV cure material, it is critical that the edgebond is, in fact, a material specifically designed for this use. Optimized material viscosity is essential to avoid the possibility of uncured material underneath the device. If the material viscosity is such that it flows too far underneath the package, UV light may not reach all of the material, resulting ultimately in device failure. Fully cured material is imperative to ensure long-term device reliability. Complete cure also can be accomplished using materials that have a secondary cure mechanism or possess the ability to cure in shadowed areas.

Figure 4A. PoP package components: Bottom package (left) and top package (right).Figure 4B. Fully assembled PoP using reflow-cured underfill for protection.

null

What’s Next

New device configurations, increased package miniaturization, and the push for more reliable and cost-effective products will continue to drive underfill technology advancements. Moving package-level thermal performance characteristics into a system that is conducive to the board-level requirements of simplified storage, reworkability, ease of use, and processability will push materials scientists for further advances in modern underfill systems. This, along with new solutions for emerging stacked package applications, will place greater emphasis on optimized processes incorporating materials that can deliver multiple functions in a single formulation. Underfill materials technology is far from mature; these materials will maintain the pace of modern electronics advancement.

REFERENCES:

1. Dan Maslyk and Brian Toleno, Ph.D., “Process and Assembly Methods for Increased Yield of Package on Package Devices,” Pan Pacific Conference, January 2008.

ACKNOWLEDGEMENTS:

1. Dan Maslyk, Brian Toleno, Ph.D., and Mark Previett, “Using Underfills to Enhance Drop Test Reliability of Lead-free Solder Joints in Advanced CSP Packages,” SMT, May 2006.
2. George Carson, Ph.D., and Michael Todd, Ph.D., “Underfill Technology: From Current to Next-generation Materials,” Advanced Packaging, June 2006.

Brian Toleno, Ph.D., director of technical service, the electronics group of Henkel, may be contacted at 15350 Barranca Parkway, Irvine, Calif. 92608.