Page 41 - Building A Succesful Board-Test Strategy

What Is a Jest Strategy? 27

type until it had produced a month's supply, about three days. Operators then
changed over to another product for a month's production, and so on, A test
strategy that worked in one location proved awkward in the other.
How often does the product change, and how does it change? Although disk
drive technology changes very frequently, the controller board's physical layout
changes more than its function. PC motherboard function also does not change
much (except for speed and memory capacity) from one generation to the next,
Large portions of a functional test would remain the same for several generations,
whereas an in-circuit test or inspection step, which relies more on the layout and
on the integration level of individual devices, would change much more.
How much of the board logic is accessible through the edge connectors'? Are
there test connectors? (Test connectors are special connectors that are theoretically
transparent to the circuit except when matching connectors are attached during
testing. The operative words are "theoretically transparent." Designers hate them
because they take up real estate, restrain the design, and may compromise circuit
performance.)
What is the mix of surface-mounted and through-hole components? Does
board assembly call for components on both sides?
Is the board mechanically stable? That is, are logic-node locations for bed-
of-nails testing known? Or are they likely to change many times in the early life of
the board? If the board is not mechanically stable, creating a bed-of-nails test in a
timely manner will be difficult or impossible.
Is the board thermally stable? Multilayer boards consist of layers made from
a variety of metallic and nonmetallic compounds. Coefficients of thermal expan-
sion may not match from layer to layer. Inside systems, during normal operation,
many boards experience temperature gradients, adding twisting moments and other
mechanical stress.
For example, it has been said that air movement inside some early personal
computers was about comparable to the air movement produced by the wings of
a dead butterfly. Therefore, a motherboard was often warmer near the power supply
than away from it. Such "hot spots" can cause the board to deform.
Although deformation is rarely great enough to bother leaded components,
which "give,"' solder around surface-mounted components may crack or the com-
ponents themselves may pop into an upright position, severing the connections at
one end, a phenomenon known as "tombstoning," as Figure 1-7 illustrates. A board
that is susceptible to tombstoning may require either a system burn-in or some
other kind of an elevated-temperature test to ensure reliability in the field.
A conventional board test may not suffice to determine susceptibility to these
thermal problems. Most board tests occur in open air or some other uniform-
temperature environment, in which an effect that results from a temperature gra-
dient will not manifest. These problems must therefore be detected using another
method—such as solder-joint integrity predictions during x-ray inspection—or
during subsequent system-level test, where the board experiences environmental
conditions identical to those in the final product.