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This section describes handling and
storage risks, and needed precautions for microcircuits. Use this section to control the environment
for which the microcircuit is used and in establishing procedures to safeguard
microcircuits.
PEM moisture
sensitivity
Moisture inside a PEM turns to
steam and expands rapidly when the package is exposed to the high temperature of
vapor phase reflow, infrared soldering, or, if the package is submerged in
molten solder, wave soldering. Under certain conditions, the pressure from this
expanding moisture can cause internal delamination of the plastic from the chip
and/or leadframe, internal cracks that do not extend to the outside of the
package, bond damage, wire necking, bond lifting, thin film cracking, or
cratering beneath the bonds. In the most severe case, the stress can result in
external package cracks. This is commonly referred to as the “popcorn”
phenomenon because the internal stress causes the package to bulge and often
crack with an audible “pop”. Surface mount devices (SMDs) are more susceptible
to this problem than through-hole parts because they are exposed to higher
temperatures during reflow soldering. The reason for this is that the soldering
operation must occur on the same side of the boards as the SMD. For through-hole
parts, the soldering operation occurs under the board, which shields the parts
from the chip or mount pad interface to the outside package surface, which has
been identified as a critical factor in determining moisture sensitivity. Test
method JESD22-A112-A, “Moisture –Induced Stress Sensitivity for Plastic Surface
Mount Devices”, and JEP113A, “Symbols and Labels for Moisture Sensitive
Devices”, can be used to assist in selecting and designing in PEMs. Inspection
for post reflow soldering often includes acoustic microscopy IAW JEDEC-STD-035
which will detect visually unseen, internal delaminations occurring as a result
of “popcorning” conditions.
All PEMs have a moisture
sensitivity level, which are levels 1 through 6 (See JEP113A “Symbols and Labels
for Moisture Sensitive Devices”). Level
1 is the least sensitive to moisture and 6 the most. Through-hole PEMs are level
1-2 and surface-mount is usually a level 5-6.
Even when the moisture sensitivity level is marked on the parts
packaging, testing and/or monitoring should be performed. Test to JESD22-A112-A
(“Moisture –Induced Stress Sensitivity for Plastic Surface Mount Devices”).
Evaluations have been performed on PEMs with sensitivity levels marked and they
tested differently than the labeled level. A number of parts marked level 1 or 2
were actually level 6, following an evaluation of the parts. Unless level 6 is
acceptable, testing-monitoring should be done until a level of confidence is
met.
Microcircuit
Storage
The discussion on storage
needs to be separated into part storage and dormant application storage.
Hermetically sealed microcircuits that were QPL, QML, or were Military/NASA
“Hi-Reliability” vintage, have been placed in storage for several years and used
successfully. It has been proven that
hermetically sealed parts can be stored as parts only, or in an application that
sits dormant for years and then works. On the other hand, PEMs have not proven
they can be stored for any given length of time. Non-military users do not
normally store parts beyond a couple of months. Therefore, the military cannot
rely on data from other industries to substantiate storage of PEMs. It has not
been proven, but PEM part storage in a temperature controlled, clean and dry
environment could probably be effective, for parts only, up to 7-8 years. Stored
dormant in an application would be a totally different story. The general concern or question is can an
application, like a missile system, sit dormant for years, maybe up to 20 years,
and then the PEM function in the application. The example used by many to
substantiate PEM usage in military applications is with Navy Sonobuoys. Although
PEMs have been used in Navy Sonobuoys for some time, the facts do not suggest
this special application qualify PEMs as reliable alternatives to standard
military microcircuits with hermetic sealed packages. First, the Sonobuoy is a
special, one-time use “throw-away” commodity not intended for the lengthy
service life of up to 20 years required by typical tactical military systems.
Sonobuoys are not stored for many years, as they are removed from service after
5 years typically (occasionally extended to 7-8 years), at which time they are
either used for training or destroyed. The Sonobuoy deployment method includes
consideration of a possible failure rate of up to 10%, which is not an
acceptable level for tactical military hardware. The design includes a storage
container with an “O” ring seal to minimize atmospheric exposure and desiccant
is used to ensure a dry environment. The Sonobuoy unit also uses “O” rings to
seal electronic compartments and a desiccant indicator is employed. “O” rings
are very effective seals under even high pressure conditions, and under normal
atmospheric pressures afford nearly a hermetic type seal. In reality, the PEMs
used in Sonobuoys are subjected to less severe environments and reliability
demands than either typical military or many commercial products. (Note: On rare occasions, failure of units
from a given lot has been as high as 20 to 40%. Also, there had been instances
where the gases eluting from the plastics resulted in circuit failures negating
the use of the Sonobuoy.)
ESD
Microcircuits must be properly
protected during test-handling insertion and all stages of manufacturing. Many
microcircuits use high-impedance input stages, particularly metal oxide
semiconductor (MOS) technology. This technology is quite sensitive to ESD. Precautions to ESD are simple. First, store
the microcircuits in some fixture that shorts all the leads together. Second,
the circuits must be handled with care, preferably by ESD trained personnel,
grounded by a ground strap during work operations. To reduce ESD at the source,
assembly and test areas should not be carpeted. The moisture in the air should
be regulated at a 40 or 50 percent humidity level. Soldering irons should be
designed to prevent electric potential to be collected or generated at the tip
during use. Grounded soldering tips may be required. Equipment containing these
sensitive microcircuits should not be handled while wires are still hanging
loose. The ESD section under Library provides background
information relative to the impact of ESD charges on microcircuit
reliability.
Glass
seal
Many hermetically sealed packages
use glass-to-metal seals as the final package closure. Since the leads and lead
frame go through this glass closure and since the glass, which is used, is not
maximized for strength, great care must be taken in handling leads. Excessive
bending of the lead or the transmittal of forces into the lead during a clipping
operation can cause the glass to develop mircocracks, which expose the internal
cavity of the microcircuit to the contamination of the outside
environment.
Lead
soldering
A common practice is to
preprocess the leads of microcircuits before soldering. Two such processes,
which lead to problems, are hot-tin dipping of the leads and chemical lead
brightening. Hot-tin dipping must be precisely controlled to have a short dwell
time, and the solder dip must not touch the body of the microcircuit or the lead
frame above the shoulder of the lead. Lack of control exposes the microcircuit,
in particular the glass-to-metal seal, to an excessive temperature charge,
causing degradation of the microcircuit internally or micro-cracking of the
glass.
Reforming
leads
The reforming of microcircuit leads
requires great care in the design of proper tools that minimize the strain
during attachment of leads to the package. In cases where it is questionable
whether any process step has damaged the hermetic-seal capability of the
package, a good practice is to run a trial sample of parts through the process
in question.
Surfaces
In using microcircuits, handle all surfaces
carefully, which are electrically connected to one of the terminals, to make sure they are not in inadvertently
energized. It is common for metallized regions of the top and bottom ceramic
dual-in-line packages to be connected to one terminal of the microcircuit. When using microcircuits, it is essential
that they not be inserted into a socket or equipment that is energized. It is essential that the ground pin be
connected firmly before any other leads have voltage current applied to
them, and that microcircuits be operated totally within the voltage-current
ratings established by the manufacturer.
PEM
temperature sensitivity
PEMs have limited operating
temperature capability (typically +159°C – the
glass transition temperature of most encapsulants) and for this reason must be
protected from the heat of soldering baths and soldering irons. Plastic packages cannot give 100 percent
protection to the microcircuit against various contamination liquids, so these
microcircuits must be sealed and stored in a relatively dry and uncontaminated
atmosphere. When active cleaning baths are used as part of the assembly process,
additional less active cleaning baths must follow to remove all traces of the
active cleaning baths, which may become a source of destruction to the plastic
microcircuit. Most plastic packages have a small area on the end of the package
through which the microcircuit chip has been mounted. It is electrically
connected to the circuit, and proper precautions must prevent bringing it into
contact with any other potential.