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Preform with Engineered Metal Mesh for Improved Bondline Control & Increased Reliability between Substrate & Baseplate in IGBT Module Assembly
Keywords: bond line control, DBC Baseplate, embedded mesh
Bondline control with spacers or standoffs applied to a power module’s baseplate to achieve a homogenous solder layer, is a well understood technology employed by most power semiconductor manufacturers. It is understood that an inhomogeneous solder layer can lead to early device failure caused by cracking and delamination of the solder during thermal cycling, and that spacer technology such as stitched wirebonds for AlSiC baseplates or stamped ‘bumps’ on copper baseplates can inhibit this behaviour and increase joint lifetime. This study presents a novel alternative method of achieving bondline control on AlSiC baseplates whilst offering a drop in solution with no additional manufacturing steps or capital investment costs; a solder preform engineered with an embedded metal mesh across the area of the preform. Samples were made to evaluate the thermal fatigue resistance of this technology in comparison to both the traditional wirebond method and samples made without any bondline control. AlN Cu substrates were soldered to 140x70mm AlSiC baseplates for these trials with each bondline variant. The samples were then temperature cycled at a ΔT of 200K, and analysed by scanning acoustic microscopy every 200 cycles. Following module assembly the top surface of the substrates underwent a surface profile scan to determine the substrate tilt. The mean co-planarity deviation revealed the sample with the embedded metal mesh had the lowest co- planarity deviation of 52.5µm and the sample with no bondline control had the highest deviation of 67.5µm. During temperature cycling, the samples with no bondline control showed cracking and delamination at 600 thermal cycles at the titled side. The samples with Al wirebonds showed cracking at 800 thermal cycles with cracks appearing at the thinner end of the solder joint. No cracks or solder delamination was seen for the samples with the embedded metal mesh even at 2000 cycles. The metal mesh within the solder preform helps suppress solder fatigue and enhanced the reliability compared to the traditional Al wire-bond stitch method. In addition, the embedded metal mesh preform offers a drop in and cost effective solution to achieving bondline homogeneity without additional process steps or costly capital investment for wire-bonding spacers (eliminates the need for dedicated wirebonders, maintenance, fixtures etc.).
Karthik Vijay,
1976
Milton Keynes, Buckinghamshire
United Kingdom


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