Hybrid integration on low-cost flex foils using photonic flash soldiering

Abstract

Soldering of packaged electronic components using industry standard Sn-Ag-Cu (SAC) lead-free solders on low-cost foils, which are often the substrate of choice for flexible electronics, is challenging. This is mainly originating from the fact that the reflow temperatures of these solder alloys are normally higher than the maximum processing temperature of the low-cost flex foils. To enable component integration on the low-cost foils a novel method for soldering has been introduced by Holst Centre as an alternative to oven reflow, termed "photonic soldering". In this method high intensity photonic flashes are used to deliver the thermal energy required for soldering. By taking advantage of the selectivity of light absorption, the required energy for soldering is delivered to the components and circuit tracks while excessive heating of the foils is avoided. This paper presents successful photonic flash soldering of packaged LED components on low-cost polyethylene terephthalate (PET) foils using conventional SAC solders as a demonstration of the capabilities of this novel soldering technology. IntroductionReliable integration of conventional electronic chips and passive components next to printed electronics on flexible foils enables realization of smart flexible systems with complex functionality [1,2]. To reduce the cost of such systems, low-cost flexible foils are preferred as the base substrate. However, reliable bonding of components on low-cost foils is generally challenging, due to limited processing temperature range of the foils. Conductive adhesives with relatively low curing temperatures (typically 120-150 °C) are commonly used for component bonding on such foils [3][4][5]. While for a variety of applications conductive adhesives provide sufficient reliability, for more demanding applications such as automotive or wearables higher quality bonds are required.Soldering has been commonly used in electronic industry for reliable component bonding on printed circuit boards (PCBs). However, the melting temperature of industry standard solder alloys such as lead-free Sn-AgCu (SAC) is above the maximum processing temperature range of low-cost foils such as polyester foils, which renders oven solder reflow on these foils practically impossible. Low-temperature solder alloys with melting temperatures below 150 °C such as SnBi alloys can be used as alternatives to SAC solders; however there are reliability challenges associated with low-temperature