Soldering of LCCs to the MIBs is a critical final step in the production of
high reliability ceramic modules. Good solderability characteristics are
achieved through solvent and aqueous cleaning, burnishing, and pre-tinning. A
thin layer of nickel and gold is sometimes applied to the solder area to
improve solder wetting and to prevent oxidation of the solder pads.
LCCs must be cleaned and pre-tinned. Visual inspection of pre-tinned parts
at approximately 10X magnification should be used to verify the acceptability
of pre-tinning. Parts should be inspected for proper wetting, damage, excess
solder, solder shorts, and evidence of leaching.
The solder pads on the MIBs are coated with flux or flux and solder in
preparation for surface mounting of the components. This is achieved via
solder screen printing, solder stencil printing, solder paste dispensing, or
Components are placed on the MIBs by both manual and automated methods. The
solder paste itself usually serves to hold the components in place until the
board can be heated to reflow (melt) the solder and form true metallic bonding
of the components to the pads.
Inspection following component attachment is one of the more difficult
problems receiving continuing attention. No substitute for visual inspection
has yet been found, although X-ray can be used to augment visual inspection,
particularly where the solder joints are out of sight beneath the components.
Stringent process control is especially critical in this technology since
inspection cannot be relied upon to screen out all manufacturing defects.
Cleaning to remove flux residues and other contaminants completes this step of
the assembly process.
Following attachment of frame, connectors, and cross-overs, testing
determines if all end item requirements of the assembly have been met. This
includes electrical testing and visual examination both before and after
environmental stress screening. Following any rework which may be required,
conformal coating applies acrylics, polyurethanes, silicones, or parylene:
epoxies are not used on ceramic modules.
Reliability Issues and Stress Testing
Potential problem areas associated with the fabrication and assembly of
MIBs using either copper or noble metal conductive materials include the
- Solder fatigue
- Mounting pad failure
- Bowing and cracking of ceramic blanks
- Brittle copper trace
- Dielectric porosity
- Cracked dielectric
In a limited sample of some 25 million reported operating hours of LCCs on
MIBs over a two-year period, only 3 failures attributable to the LCC-on-MIB
packaging and interconnection technology were reported. Two of these were MIB
shorts at installation. Other failures, including integrated circuit
functional failures, were not associated with this technology.
emphasis of stress testing relates to the integrity of the solder joints
attaching the LCCs to the MIBs and the ability of the MIBs to withstand the
effects of moisture. Temperature cycling, temperature shock, and power cycling
are recommended as the best screening tests for MIB assemblies.