It is recommended that a limited range of package styles and
sizes be used for ease of both design and producibility. The preferred package
style for ceramic MIBs is the ceramic leadless chip carrier (LCC) for packages
with low pin counts. For high pin counts, ceramic leaded chip carrier (LDCC)
packages should be considered. The type of LCC to be used is specifically the
JEDEC three-layer package. Single layer packages should not be used due to
reliability problems. Typically, 50-mil-pitch packages may be used for pad
counts less than 84. For larger packages (84 pads and greater), 40-, 30-, 25-
or 20-mil pitch is recommended to limit the package's physical size.
For parts not available in LCC configurations, other
surface-mounted packages are acceptable such as ceramic flatpacks. Using these
parts, however, will cause greater difficulty in assembly and rework if they
are mixed with LCCs on the same board. It is recommended for ease of rework
that ceramic gull-wing flatpacks be used for large parts not available in LCCs
such as digital-to- analog converters.
Most discrete resistors and capacitors are available in ceramic
chip form and should be used rather than axial-leaded devices for ease of
assembly and rework. There are several types of resistor and
resistor-capacitor networks available in LCCs which can be used instead of
discretes to save board area.
Proper thermal design is essential to the reliable operation of
a MIB assembly. The failure rate of electronic components has been shown to
increase exponentially with junction temperature, as is predicted by failure
models. A failure rate decrease of 20% to 50% for a 10=C reduction in junction
temperature is usually predicted. In addition to the thermal constraints on
electronic components, there are thermally induced failure modes in electronic
packages. such as thermal stress due to a difference in the thermal
coefficient of expansion and localized degradation of encapsulants or
materials due to component hot spots.
Early in the design phase, the designer must
consider the component-level and system-level thermal requirements. These must
be reflected in the placement of the parts, the selection of materials for the
MIB and its support structure, the use of heat sinks and thermal aids, and the
method of installation of the MIB into the system. The data shown in Figure
presented as an aid for thermal considerations during the preliminary design
phase. It presumes that the LCC package size has been properly chosen for the
die size and shows the typical measured thermal resistance from the IC
junction to the LCC solder bumps. The MIB design must provide an acceptable
thermal impedance from the LCC solder bumps to the heat sink arrangement to
meet system requirements. The following thermal considerations are recommended
to keep IC junction temperatures to a minimum:
• Plastic-packaged ICs have poor thermal transfer and high
porosity--they should be avoided.
• A metal or ceramic package using eutectic die bonding is
• Locate higher-powered ICs as near to the ultimate heat sink as
possible to minimize thermal resistance.
• ICs with higher failure rates due to complexity or structure
should also be located near the ultimate heat sink.
• When custom packaging chips, use the smallest package
compatible with chip size and pin count to minimize thermal resistance.
• When necessary, thermal pads may be used on
the bottom of the LCC package and solder-connected to a matched pattern on the
MIB (see Figure
4-15). For large LCCs, the thermal resistance could be reduced as much as 3 or
4 to 1 when using thermal pads.
CAUTION:Thermal pads will complicate component removal. Prior to
production release, the designer should verify either through test or through
detailed thermal analysis that the resulting operating temperatures of the
module meet the criteria established for the design.
SPACING AND ORIENTATION
For ease of assembly, inspection, and test, it is
recommended that pin #l of all components, both active and passive, be
oriented in the same direction. There should be no less than 0.050-inch
spacing between packages, except for packages one inch or greater in size,
where 0.100-inch spacing is recommended as the minimum spacing between
components. (These spacing recommendations are based on the maximum carrier