Solder Alternatives for the Transition to Flexible and Integrated Electronics

Solder Alternatives for the Transition to Flexible and Integrated Electronics The attachment of electronic components such as integrated circuits and LEDs to substrate layers is an important aspect of electronic manufacturing. While traditional lead-free solder performs well for conventional rigid PCBs, it is not well-suited for new emerging applications that require bonding components to flexible substrates or compliant surfaces.

Current State of Component Attachment

Component interconnection materials can be divided into two categories. Formerly lead-based, but now primarily lead-free solder, offers two advantages: low cost and established use. Additionally, the reflow process helps with component alignment through repositioning of components to minimize the surface energy of the solder droplet. This self-alignment significantly reduces the resolution requirements of the pick-and-place process, enabling higher-speed component placement. However, solder has one significant disadvantage. The reflow process typically requires high temperatures around 250^oC for commonly used SAC (tin/silver/copper) solder and also requires time-consuming thermal ramps. For this reason, it is not suitable for thermally fragile substrates such as PET and is incompatible with high-speed roll-to-roll (R2R) production or 3D additive electronics. It should not be forgotten that ultra-low temperature solders have recently been developed to improve compatibility with polymeric substrates, but have not yet gained widespread acceptance. The high temperature and time-consuming reflow process is discussed in detail in IDTechEx's new report "Electrically Conductive Adhesives 2022-2032: Technologies, Markets and Forecasts," which covers electrically conductive adhesives (ECA). Instead of being a solder-like metal alloy, ECAs contain metal powder embedded in a polymer resin. Despite being more expensive and lacking the self-alignment advantage, ECAs allow immediate bonding of components to the substrate and do not require high reflow temperatures or lengthy thermal ramps. For this reason, they are unlikely to replace solder for conventional PCBs, but their market share is expected to increase with the transition to integrated and flexible electronics. Additionally, anisotropic conductive adhesives (ACAs) provide lower pitch capability than the more common isotropic conductive adhesives (ICAs). Currently, ACAs (supplied as films or pastes) require high temperature and pressure to be applied during component placement, which increases both manufacturing costs and the risk of component damage. However, the emerging 'field-aligned conductive adhesives' technology eliminates this constraint since anisotropic conductivity is induced before bonding.

Emerging Applications for ECAs

ECAs are particularly well-suited for applications requiring electronic components to be mounted on something other than a conventional FR4 substrate, especially when the substrate material is thermally fragile. A promising application for ECAs is in-mold electronics (IME), used to create decorative surfaces with integrated capacitive touch sensing and lighting. Automotive interiors are the primary use case, with the total IME market estimated to reach approximately USD 1.5 billion by 2032. IME components are produced by screen-printing conductive ink onto a flat (typically polycarbonate) substrate and then mounting electronic components. ECAs are used because polycarbonate cannot withstand high solder reflow temperatures. Another promising ECA application is flexible hybrid electronics (FHE), an emerging approach that can be considered as taking "the best of both" conventional and printed electronics. This requires mounting components on flexible substrates with printed conductive traces using ECAs (or alternatively ultra-low temperature solder) to avoid damaging low-cost PET substrates. A very specific use case for ECAs is stretchable electronics, required for e-textiles and some electronic skin patches. This will require somewhat stretchable component attachment solutions. Since the mechanical properties of ECAs depend largely on the properties of the polymer resin, slight stretchability can be achieved by using an elastomeric material. Source