Independent of any lower-layer electrical test plan chosen by
the MIB manufacturer and/or customer. 100% electrical testing should be
performed on each completed lot of MIBs. MIB electrical evaluation consists of
isolation and continuity testing which can be performed using a computerized
database to memorize or "self learn" multiple points among designed nets. The
complex multipoint networks, which by design are either isolated or connected
within a conductor path or routing, usually necessitate automatic testing for
expediency over a more time-consuming manual probe.
A number of automatic test methods have been developed for
performing continuity and isolation testing. Perhaps the most common method
utilizes automatic test equipment including a forced direct current source
with current-measuring capability. The correct node/net array having
previously been programmed, any deviation from the standard results in the
identification of an electrical fault. i.e.. an "open" or "short." Once a
fault location is identified, manual probe confirmation may be performed. A
"bed of nails" test fixture can rapidly perform isolation and continuity for
up to 2000 or more test points. However, close-spaced component pad designs
(e.g.. 0.025 inch) can pose alignment problems. Other methods using "flying
probes" with either AC or high voltage DC have also been developed.
The equipment used to perform the MIB testing for opens and
shorts should be designed so as not to ''blow out'' shorts that might be
encountered between traces. A typical value of power limitation is 50 mW.
Experience of a number of MIB manufacturers and users has been that blown
shorts may regrow in time.
In order to assure reliability, a MIB must withstand the stress
of changing temperature without developing open or shorted circuits.
Temperature cycling is an accepted procedure for screening manufacturing
defects. If the defect rate is extremely low, it could be combined with
assembled MIB (module) temperature cycling--most easily accomplished if MIB
fabrication and assembly are performed within the same facility. Because
copper has a known failure mechanism that can be detected by temperature
cycling, copper MIBs must be verified before component assembly; fault
detection after assembly is too expensive.
Electrical verification is required before and after MIB thermal
cycling ( 10 cycles, -55 to + 100 =C). If no failures are found, the lot is
accepted. If a failure is detected, the discrepant part(s) is removed and
thermal cycling of the lot is repeated (10 cycles). If no further
discrepancies are found, the lot is passed. After three sets of thermal
cycling (a failure in each set), the lot is subject to rejection. Continuity
rejection limit should be 10 ohms or less. Metallographic investigation of the
internal grain structure along with ductility testing, i.e., multiple 90=
bends of freed lines or pull testing, should be used to verify the existence
of a failure mechanism and thereby determine the disposition as use or scrap.
Metallographic verification of the structure, if performed as a lot sampling
along with temperature cycling, is a good procedure and can supplant cycling
once a comparative baseline has been established.
Final visual inspection of completed MIBs is performed to
determine compliance with customer-specified criteria and workmanship
A completed MIB top layer consists of component mounting pads,
I/O pads and other external solderable surfaces. Grid planes (Vcc, ground),
internal signal layers, via fills, and dielectric layers are inspected during
the fabrication of the MIB and are not within the scope of final visual
Final visual inspection, as a screening
element of the acceptance test procedure, is performed by the manufacturer.
When specified by contract, government source inspection may also be required.
Table 5-3 contains general visual guidelines.
Standard visual inspection conditions include optical apparatus
providing minimum specified magnification, side lighting, as appropriate, and
specified criteria clearly defined. Visual aids for clear criteria definition
would greatly reduce inspection errors for subjective, incorrect
Many manufacturers are moving toward automatic inspection
equipment in anticipation of repeatable objectivity and reduction of
inspection cycle time.
Finger cots/lint-free gloves should be worn by all inspectors to
The effects of humidity on multilayer thick film boards are
indicative of two different mechanisms, surface effects and bulk effects.
Under high humidity conditions, a film of water condenses on the
surface and is absorbed into the bulk of the dielectric through any porosities
or cracks. Use of a surface glaze on recent MIBs has succeeded in greatly
reducing surface porosity and therefore absorption.
The presence of water on and in the dielectric results in
increased electrical leakage when voltage is imposed. Copper MIBs tested under
electrical bias during humidity conditions have sometimes shown what appears
to be permanent reduction in insulation resistance. In large part, this is a
surface effect caused by electromigration of metal ions or salts through
surface moisture films. These surface deposits remain slightly conductive even
after thorough drying of the sample. When these deposits are removed, however,
good dielectric quality is restored.
A second more damaging effect occurs if surface contamination
such as flux residues or perspiration salts from fingerprints are present. Any
moisture penetrating into porosities or cracks will carry these salts into the
bulk of the dielectric and may cause increasing leakage within the dielectric
When testing MIBs for insulation quality under moisture
conditions, it is imperative that the surface be free of ionic
Additionally, when biased humidity testing is performed, closely
spaced surface conductors, which are at different electrical potentials,
should be protected by coating the area with a suitable insulating material
such as silicone rubber, epoxy, or polyurethane-based conformal coating. This
will prevent formation of surface metallic films and allow a truer evaluation
of dielectric body quality.