The newly-developed low-melting-point glass
The newly-developed low-melting-point glass
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Hitachi Ltd and Hitachi Chemical Co Ltd co-developed low-melting-point glass that can be used for bonding at a temperature of 220 to 300°C.

They started sample shipment Nov 26, 2012, planning to begin volume production in 2014. Hitachi Chemical aims to achieve sales of ¥2 billion in 2015 with its low-melting-point glass adhesives including the newly-developed one.

The companies developed the glass by using vanadium (V), silver (Ag), tellurium (Te) and oxygen (O) as main elements and adding small amounts of phosphorus (P), barium (Ba) and tungsten (W). And it does not contain halogens that are subject to RoHS, REACH and other environmental regulations such as lead (Pb) and fluorine (F), which is volatile. Because the new glass has a oxide glass structure, it can be used as an adhesive in air, nitrogen and vacuum.

As adhesives that can be used at a temperature of 220 to 300°C, there are AuSn (gold-tin) solders and epoxy-based heat-hardening resins. Compared with AuSn solders, the new glass adhesive has a lower cost, Hitachi Chemical said. Also, it has a higher airtightness and moisture resistance than epoxy-based heat-hardening resins.

Furthermore, the glass adhesive has characteristics that AuSn solders and epoxy-based heat-hardening resins do not have. Specifically, it can be used for bonding materials having different thermal expansion coefficients such as metals, ceramics and resins and supports bonding with a laser in addition to normal heat treatment.

The Hitachi Group plans not only to apply the new glass adhesive for the packaging of crystal oscillators and MEMS devices but also to develop new applications by making the most of its properties.

Melting point lowered by 100°C

In December 2009, the Hitachi Group developed low-melting-point glass that can be used for bonding at a temperature of 350 to 400°C. Hitachi Chemical started to mass-produce it in February 2012. And the glass has been employed as an adhesive for the packaging of crystal oscillators and MEMS devices.

The low-melting-point glass developed in 2009 has a structure that uses V, P and O as main elements. With this glass structure, however, it is difficult to lower melting point due to its strong connection with O ions. And reliability is lowered because PO4 in the structure is bonded to water molecules due to the ingress of water.

Therefore, for the old low-melting-point glass, Te, whose connection with O ions is weak, is used in its glass structure to lower melting point. And Ba, which has a large ionic radius, is used to prevent the ingress of water, which lowers reliability.

This time, Hitachi and Hitachi Chemical made two improvements to further lower melting point while keeping reliability. First, to lower melting point, they used Ag, which has a weaker connection with O ions than Te. And they reduced the amount of PO4, which lowers reliability with the ingress of water.