Page 84 - Building A Succesful Board-Test Strategy

70 BUILDING A SUCCESSFUL BOARD-TEST STRATEGY

20 mil

100 mil

Figure 2-13 In surface-mount fixtures, a series of translator pins pass through a center
fixture platen so that at the board the pins can be much closer together.

that at the board, the pins can be much closer together, as Figure 2-13 shows. The
principle here is that even on heavily surface-mount boards, not all nodes require
very close centers. The center platen minimizes pin bending and therefore improves
pointing accuracy.
This type of fixture is about 20 to 50 percent more expensive than a conven-
tional type. In addition, if too many areas on the board contain close centers, the
number of available tester pins may be insufficient, or wires from the receiver may
be too long to permit reliable testing.
Each of these fixture designs assumes that the tester requires access to only
one board side at a time. This approach means either that all test nodes reside on
one side (usually the "solder" side) or that board design permits dividing the test
into independent sections for each side. This latter approach, of course, means that
each board requires two fixtures and two tests. In many manufacturing operations,
however, capacity is not at issue, so that testing boards twice per pass does not
present a problem.
Testing both board sides simultaneously requires a clamshell fixture that con-
tains two sets of probes and closes onto the board like a waffle iron. Clamshell fix-
tures are extremely expensive. A test engineer from a large computer company
reported that one of his fixtures cost nearly $100,000. To be fair, the board was
18" x 18" and contained several thousand nodes. Nevertheless, fixture costs that.
approach or exceed tester costs may necessitate seeking another answer.
Clamshell-fixture wires also tend to be much longer than wires on a conven-
tional fixture. Wire lengths from the receiver to the top and bottom pins may be
very different, so that signal speeds do not match. If a pin or wire breaks, finding
and fixing it can present quite a challenge. The rat's nest in one of these fixtures
makes its conventional counterpart look friendly by comparison.
The accuracy, repeatability, and reliability of a clamshell fixture's top side are
poorer than those of a conventional solution. Access to nodes and components on
the board for probing or pot adjustment is generally impossible. In addition, the