An electrical spring probe has an elongated contact and a helical spring.
The spring probe is mounted in a through hole of a non-conductive
substrate. The elongated contact includes a head with a V-slot groove for
engaging a solder ball lead of an IC package, a shoulder providing a
surface to retain a helical spring and a beam for providing a short
contact path through the spring. The helical spring is disposed about the
contact, with contiguous coils on each end. The upper end of the spring
is secured to the contact shoulder immediately under the head. The middle
coils of the spring have a larger diameter than the contiguous coils on
either end to retain the spring probe assembly in the non-conductive
substrate. The bottom end of the helical spring has contiguous coils some
of which extend below the substrate surface to make electrical contact
with a printed circuit board. The bottom contiguous coils are of a
reduced diameter and have a center axis offset from the main axis of the
spring probe assembly. When the spring probe is compressed between the IC
package and the printed circuit board, the offset coils deflect the beam
of the contact into the bottom contiguous coils establishing a direct
electrical path between the IC package and the printed circuit board with
minimum electrical impedance.