Soldering terminology

PS is the abbreviation for pressure soldering. This method of soldering, developed by Tokuda, Chiba and Mizuishi at the Hitachi Research Center, offers two main advantages: (1) no flux is needed; (2) pore formation is excluded. It is best explained using the example of a silicon pellet which is soldered to a substrate (see figure). The contact surface was previously metallized for this purpose.

1. The Si - pellet is placed on the substrate, together with a pre-formed solder deposit serving as an intermediate layer. Ambient pressure is reduced to P₁ (about 1 Torr). As there is a natural gap on the contact surface, pressure P₁ also prevails in the interior of the solder preform.

2. The gap disappears when the solder is melted and the interior of the solder preform is hermetically sealed at P₂ .

3. If the external pressure is reset to the normal P₂ pressure (760 Torr), the pressure difference (P₂ -P₁) arises between interior, sealed airtight at P₁, and the exterior and the molten solder flows immediately into the cavity of the solder preform.

4. The heat supply is interrupted and the solder solidifies.

If the volume and temperature values measured inside the solder preform at the time of executing steps (2) and (4) are represented as V₁, V₂ and T₁ ,T₂ it follows from the Boyle-Charles’ law: