PEMs_Use

Early Usage and Advancement

PEMs Early Usage

Due to high failure rates, early PEMs usage in the 1970's was discouraged. Common failures for PEMs included3:

Failure Cause
Wire bond intermittency/lifting
Wire/metallization corrosion
Voiding/poor adhesion
Data/soft errors
Coefficient of thermal expansion (CTE) differences
Moisture/contamination
Processing/materials
Alpha particles (filler material)

Advancement

Major improvements in the following areas have led to acceptance of the use of PEMs in the Aerospace and Defense industry:

  • materials
  • material attributes and characteristics
  • lead frame design
  • die coatings
  • die design
  • fabrication equipment
  • and testing procedures3

These improvements have led to the increase in the reliability of PEMs in these areas:

  • reduced phosphorus levels in passivation
  • dual layer passivation in critical cases
  • perforated frames
  • benign (non-ionic) cleaning of frames after molding
  • use of copper frames
  • reduced stress trim and form
  • corrosion resistant mold compounds
  • nitride passivation
  • control/elimination of ionic contamination
  • comprehensive reliability program

Remember that use of PEMs for one application does not ever qualify it for use in another applications without proper review, regression testing, etc.

Broad Considerations
Concerns associated with the increased usage of PEMs, especially military usage, include:
  • Uncertainty regarding their long term reliability in harsh environments
  • Lack of reliability/quality assurance procedures
  • Insufficient military environment reliability data (operating and storage)3
  • Existing OEM procurement expertise

Lot Concerns: One of the critical factors concerning PEMs is that their reliability may be lot dependent.6 This leads to individual lot screening.

Storage Considerations
Shelf Life: PEMs with manufacture dates older than 3 years before the time of installation shall not be used without proper approvals.4

PEMs must be stored under temperature-controlled, clean conditions, protected from ESD and humidity.4

Moisture: All PEMs have levels of moisture sensitivity.6 This should not be overlooked during storage considerations.

Handling Considerations
APL employs a four-step approach to handling PEMs. These precautions help prevent corrosion. Handling considerations call for the use of:
  1. finger cots and gloves,
  2. moisture protection,
  3. conformal coating,
  4. and adherence to electrostatic dischage (ESD) sensitivity practices and procedures.7

Product Assurance
Primary elements of the product assurance system are:
  1. Screening
  2. Qualification

Qualification consists of Radiation Hardness and Destructive Physical Analysis (DPA). Note: DPA may or may not be considered as part of Qualification Testing.

Screening Considerations
PEMs Lot Screening consists of three functions:
  1. Electrical verification (at the mission temperature profile)
  2. Radiographic examination
  3. Visual and mechanical inspection7

All screening tests except for mechanical inspection are performed on a 100% basis.7 Additional Information on PEMs qualification will be provided later in the module.

Product Assurance Relationship
A relationship between the major elements of the product assurance system and reliability of the PEMs can be illustrated using a classic bathtub-shaped curve of the lifespan failure rate.

Derating Considerations

Derating is the method of reducing stress and/or making numerical allowances for functional degradation in microcircuit performance. When using microcircuits of different temperature ranges, reliability levels, and performance characteristics, it is crucial to derate them properly.

Two methods of derating are:

  • reducing heat and electrical stress
  • compensating for functional loss