Page 86 - Building A Succesful Board-Test Strategy

72 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

Contact Head

Plunger

Probe Body

Plunger Body
Crimp Does Not
Ride on Plunger Body

Spring

Spring Is Wedged in the
Closed Area of Probe Barrel
Bottom of Probe
Barrel Is Open to
© Allow Assembly
to Breathe
Figure 2-14 Construction for a typical bed-of-nails probe. (Reprinted with the
permission of Interconnect Devices, Kansas City, Kansas.)

trating an oxide layer with a star tip requires higher spring force than with other
designs, but the star provides more contact points.
Other probe tips offer advantages, as well. Concave tips accommodate long
leads and wire-wrap posts. A tendency to accumulate debris, however, makes these
tips most effective in a "pins down" position.
Spear tips, which are usually self-cleaning unless they contain a flat spot or
large radius, can pierce through surface films on solder pads. As a general solu-
tion, however, Mawby (1989) cautions against probing holes larger than half the
diameter of the spear-tip plunger.
Flat tips and spherical-radius tips work well with gold-plated (and therefore
uncorroded) card-edge fingers. Convex designs apply to flat terminals, buses, and
other solid-node types. These tips are not self-cleaning when probing vias.
Flex tips can pierce conformal coatings, as on many military boards. Of
course, these boards require recoating after test. Serrated tips work well for access
to translator pins or on terminal strips that contain some surface contamination.