應用於軟性印刷電路板表面貼焊技術之超低回焊溫度液態金屬基底焊料

Abstract

隨著對於具有高度人體相容性的穿戴式裝置的需求增加，對軟性印刷電路板（FPCB）的需求也持續增加。然而，傳統焊接工藝的高操作溫度會對多數軟性高分子基材與部分電子元件帶來熱損傷，也會造成多層封裝結構中焊點再融化的問題。因此，在低溫下仍具有流動性與基材潤濕性的液態金屬（LM）作為傳統焊接合金的替代方案被提出，透過固液相擴散機制（SLID bonding）接合金屬基材與元件金屬接腳。但液態金屬焊料同時也存在一些問題，包括液體滲漏和低機械性能等等。為了解決上述問題，本研究引入了一種仿汞齊合金的方法來實現液態金屬焊料的完全固化與機械性能強化。本研究通過將紅銅粉末與鎵銦錫（GaInSn）液態金屬混合，直至銅粉完全被液態金屬濕潤，來製備GaInSn/Cu複合焊料。GaInSn/Cu複合焊料在低溫下（70℃）即自發性發生化學反應，在內部產生固態的鎵銅與錫銅界金屬化合物，以及銦錫固溶體，達成焊接層固化。DSC分析顯示，在固化反應後，GaInSn/Cu複合焊料的熔點從約61.4℃提高到約106.3℃。證實了本研究的製程成功解決了焊點液體滲漏問題。為了優化GaInSn/Cu複合焊料，提升焊接點機械強度與縮短焊料固化時間，本研究參考了Ga-In-Sn三元相圖來調整GaInSn/Cu複合焊料的組成。從固化後的焊接點剪切焊接強度結果顯示，其剪切強度達到約19.5MPa，比組成調整前的的強度高出三個數量級。與其他液態金屬焊料研究[1]相比，本研究的焊料在小30℃的操作溫度下，達成約3倍的強度。此外，試與塗佈了拉伸導電膠的軟性基板PET及PE纖維進行低溫焊接製程，經過功能性與彎折拉伸耐受性測試，驗證本研究開發的GaInSn/Cu複合焊料在低溫焊接製程的可行性，推進了FPCB的表面貼裝技術。

As the demand for wearable devices with high human compatibility increases, so does the demand for Flexible Printed Circuit Boards (FPCBs). However, the high operating temperatures of traditional soldering processes can cause thermal damage to most flexible polymer substrates and some electronic components, and can lead to re-melting of solder joints in hierarchy packaging structures. Therefore, as an alternative to conventional solder alloys, liquid metal (LM) with fluidity and reactive-wettability at low temperatures has been proposed. This approach, known as Solid-Liquid Interdiffusion (SLID) bonding, bonds metal substrates and electronic components using LM, avoiding the issues associated with high-temperature soldering. However, LM soldering composite also presents challenges such as liquid leakage and low mechanical performance. To address these issues, this study introduces amalgamation-inspired method to achieve complete solidification and enhanced mechanical properties of LM soldering composite. By mixing copper powders with GaInSn until the copper powders are fully wetted by LM, the GaInSn/Cu soldering composite is prepared. The GaInSn/Cu soldering composite undergoes a spontaneous reaction at low temperatures (70°C), forming Ga-Cu and Sn-Cu intermetallic compounds and In-Sn solid solution, thereby solidifying the solder joint. Differential Scanning Calorimetry (DSC) analysis shows that after solidification, the melting point of GaInSn/Cu soldering composite shifts from 61.4°C to 106.3°C, confirming the successful prevention of solder liquid leakage issues. Additionally, X-ray Diffraction (XRD) analysis verifies the compositional changes within the GaInSn/Cu composite solder. To optimize the GaInSn/Cu soldering composite for improved shear strength of solder joints and shortened solder solidification time, this study adjusts the composition of GaInSn/Cu soldering composite based on the Ga-In-Sn ternary phase diagram. The shear strength of solder joints in this study reaches approximately 19.5 MPa, which is three orders of magnitude higher after composition adjustment. Compared to the LM solder in other studies, the shear strength of solder joint in this study achieves about three times higher with an operating temperature 30°C lower. Furthermore, the feasibility of the GaInSn/Cu soldering composite in low-temperature soldering processes is validated through functional and endurance tests on flexible substrates such as PET (Polyethylene Terephthalate) and PE (Polyethylene) fibers coated with stretchable conductive adhesive. The result confirms advancements in surface-mount technology for FPCBs driven by this study.