Development of burn-in test socket for 0.5 mm 256 pins LGA

We have developed a burn-in test socket for LGA in which IC electrode-side contacts are arranged at 0.5 mm pitch and the arranging pitch of printed board-side contacts is fanned out to 1.27 mm. The fan-out is accomplished by using a lead frame that is made of a 0.1 mm thick beryllium copper sheet. The contact force of the contacts is adjusted to 5 gm/0.3 mm by using an arcuate spring. The initial contact resistance is 12 m Ω, but the resistance remains at 30 m Ω moreover after the socket is held at 125°C for 1000 hr and repeatedly bent 5000 times