Abstract
In this study, electroless-plating of a nickel-phosphor (Ni–P) thin film on surface-controlled thermoelectric elements was developed to significantly increase the bonding strength between Bi–Te materials and copper (Cu) electrodes in thermoelectric modules. Without electroless Ni–P plating, the effect of surface roughness on the bonding strength was negligible. Brittle SnTe intermetallic compounds were formed at the bonding interface of the thermoelectric elements and defects such as pores were generated at the bonding interface owing to poor wettability with the solder. However, defects were not present at the bonding interface of the specimen subjected to electroless Ni–P plating, and the electroless Ni–P plating layer acted as a diffusion barrier toward Sn and Te. The bonding strength was higher when the specimen was subjected to Ni–P plating compared with that without Ni–P plating, and it improved with increasing surface roughness. As electroless Ni–P plating improved the wettability with molten solder, the increase in bonding strength was attributed to the formation of a thicker solder reaction layer below the bonding interface owing to an increase in the bonding interface with the solder at higher surface roughness.
Highlights
Owing to their high performance at temperatures below 200 ◦ C, Bi–Te-based thermoelectric materials have been widely applied for active cooling using the Peltier effect and for power generation using the Seebeck effect [1,2,3,4,5,6,7,8,9,10,11]
In the case of the thermoelectric element block prepared without sand blasting, the Ni–P plating layer on the surface was thermoelectric element block prepared without sand blasting, the Ni–P plating layer on the surface delaminated (Figure 1a)
As there was no chemical bonding at the interface betweenand thethe plating layer, the adhesion of the electroless plating layer could be secured via the physical thermoelectric element and the Ni–P plating layer, the adhesion of the electroless Ni–P plating layer surface
Summary
Owing to their high performance at temperatures below 200 ◦ C, Bi–Te-based thermoelectric materials have been widely applied for active cooling using the Peltier effect and for power generation using the Seebeck effect [1,2,3,4,5,6,7,8,9,10,11]. The thermoelectric phenomena can be realized by fabricating thermoelectric devices/modules in which several tens to hundreds of n-type and p-type rectangular thermoelectric elements are electrically bonded in series on top of a copper electrode formed on the ceramic substrate [12]. Solder materials using Sn–Ag–Cu-based alloy, which has a melting temperature of approximately 220 ◦ C, have been widely used for bonding Bi–Te-based thermoelectric elements and copper electrodes. Sn-based solder and Te materials generate brittle Sn–Te-based intermetallic compounds at a temperature of approximately 250 ◦ C [13,14,15,16,17]. Many studies have reported that these Sn–Te-based intermetallic compounds deteriorate the bonding strength of Coatings 2019, 9, 213; doi:10.3390/coatings9030213 www.mdpi.com/journal/coatings
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