Plating chemistries are a critical piece of advanced semiconductor packaging. That said, different materials lend themselves to different applications. For example, Dow’s SOLDERON™ tin-silver (SnAg) plating chemistry has made a name for itself as a reliable replacement for tin-lead bumping in advanced packaging applications with the ability to plate across a wide range of processes from traditional C4 bumps to µbumps and capping for Cu pillars. As flip chip, Cu pillars, and µ-bumps for die stacking using through silicon via (TSV) technologies extend to new applications spaces, SnAg may not be a suitable or ideal bumping chemistry, driving a need to explore new materials. SOLDERON™ BP IN 1000 indium is one result of this work and delivers a new, indium-based plating chemistry targeting a new range of device packages. In this interview with Dow’s Lucy Wei, product marketing manager, and Yi Qin, project leader, we explore the capabilities of the latest SOLDERON™ product, and learn how it is enabling innovations in these new device designs.
Q. What new advanced packaging applications are posing a challenge for tin-silver?
Wei: SnAg will continue to be the go-to plating chemistry for advanced packaging technologies targeting logic and memory devices. However, emerging applications in optoelectronics, displays, backside-illuminated CMOS image sensors, bio-sensing, LEDs and other photonics devices, including those fabricated using III-V materials rather than Si, require alternatives for bumping processes. This is primarily due to sensitivity to the high processing temperatures of SnAg, and its impact on substrate warpage, stress, temperature-sensitive materials in the device and ultimately the device reliability. In these cases, plating with pure indium (In), which has a much lower melting point than SnAg, is proving to be a better option.
Q. What makes In a good alternative to SnAg in plating chemistries for applications where SnAg is not suited?
Yi: With a melting point as low as 156ºC, one of the most significant differences between In and SnAg is the ability of pure In to achieve low-temperature reflow. For example, a peak temperature of 180ºC is needed for In solder reflow as opposed to 260ºC for SnAg. At this significantly reduced reflow temperature range, we’ve demonstrated macro and micro void-free performance with In chemistry on nickel and copper, even after multiple reflows. This indicates that, as is the case with SnAg, reliable interconnections are being formed on under-bump metallization and pillars, which is a must for any solder material used in these types of applications. Additionally, the In chemistry has demonstrated stable and consistent performance over thermal and electrolytic bath aging, an important and favorable attribute from a process standpoint.
When we developed our SOLDERON™ BP TS 6000 tin-silver chemistry, we focused on providing as much versatility as possible to enable our customers the ability to extend the single formulation into many applications. Packaging for temperature-sensitive electronic devices are also likely to have a variety of approaches. With this in mind, we have designed SOLDERON BP IN 1000 indium to serve a wide group of applications, from C4 bumps for conventional flip-chip processes, to capping for Cu standard pillars and µ-pillars, and µ-bumps in 3D stacking using TSV processes. It is also compatible with mainstream photoresist materials, and can be integrated directly into existing process flows. It has demonstrated good within die co-planarity in test vehicles during bumping and capping applications. Additionally, its low melting temperature also makes it a good option for bonding multiple thin layers, enabling new bonding schemes.
Q. How does SOLDERON BP IN 1000 enable emerging applications that were previously limited due to material choice?
Wei: The low processing temperature of SOLDERON BP IN 1000 indium is making it possible to use bumping and bonding processes in applications where it was previously not possible. For example, low-temperature processing prevents warpage in coreless substrates, allowing for reduced substrate thickness without increasing stress, thereby enabling flexible displays. It can also reduce warpage and stress for 3D Si wafer stacking. For bio-sensing devices, low-temperature reflow processes are less likely to damage the materials inside the device. It also enables bumping and bonding processes in devices that contain components with a significant difference in the coefficient of thermal expansion.
Q. Is SOLDERON BP IN 1000 Indium intended to replace SOLDERON BP TS 6000 Tin-Silver?
Wei: No. As with most material sets, where something works well and is designed into the process of record, there’s no reason to switch to a new version. Rather, SOLDERON BP IN 1000 indium was designed to extend the SOLDERON family into new applications where SnAg can’t be used due to the thermal budget. This means we are providing our customers with much broader solder options as they develop their applications. New technologies often require materials evolution, and that is the case here. We’ve developed this material so that interconnect processes that have proven to be enabling and extremely reliable in high-volume manufacturing could be adapted to new applications. So far, we have had good results with meeting design targets for our customers in test vehicles. At Dow, we are excited about the new doors SOLDERON BP IN 1000 indium is opening for future applications.
At ECTC 2016, we presented the initial results of our work using an early generation of indium chemistry. View that presentation here.
Learn more about our new SOLDERON BP IN 1000 Indium.