Workmanship_RequirementsAndQualityComparisons

NASA defines the following Workmanship Requirements Categories [10].

Design Requirements: Controls, materials, and configurations (e.g. dimensions, placement, interface materials) are selected to provide operational performance.

Workmanship Examples: Solder material, flux material, staking of wire runs to enable performance in shock/vibration environment.

Processing Requirements: Controls the manufacturing methods or techniques.

Workmanship Examples: Use of certain type of container to mix polymers to avoid contaminating the polymer. Periodic alloy check of solder pot to ensure material purity and the control of environmental conditions such as humidity.

Defect Criteria (aka accept/reject criteria, quality criteria): Physical attributes that show evidence of a defect or are known to be indicative of the presence of a defect that will result in premature failure.

Workmanship Examples: Solder joint appearance, presence of extraneous material, nicks and scrapes in conductors, missing material and delaminated material are all examples of worksmanship.

Training and Certification of Operators, Inspectors, and Instructors

 

Review your memory then:

 

The NASA Workmanship Standards have always been intended to be implemented by the Operator and the Inspector.
Historically, design and processing decisions were made by technicians using experience and corporate knowledge. Miniaturization challenges this approach.
NASA Workmanship Standards capture some of the well-known and accepted design and processing practices that are used today.

  • Some professionals argue that a designer needs to provide the information

The trend in the Workmanship Standards is to eliminate design requirements and avoid dictating process development. Currently the industry is leaning toward operator and inspector requirements only. 

Examples of design requirements being eliminated [10]:

• High strength copper alloy is required for wires gauge 24 and smaller.
• Line voltages shall be limited to socket contacts (for safety).
• Materials to meet NASA-Std-6001, flammability, odor, offgassing
• Design of harnesses shall minimize RFI/EMI.

 

Examples of “reminders” to designers and process engineers that will be removed [10]:

• Staking materials and locations must be defined on the engineering documentation
• Conformal coating material must be defined. Conformal coating materials with a fluorescent indicator are preferred
• Bond line requirements must be defined.
• Conformal coating maskant material and areas to be masked on PCBs must be defined
• Demoisturizing conditions for PWAs prior to polymeric applications (time, temperature, ramp rates) must be defined
• Harness design must plan use of heat-shrinkable sleeving, stress relief, methods for cable identification, preventing mis-mating of connectors

 

Quality vs Reliability

Will assemblies with Workmanship defects necessarily fail?

  • Use of the materials and configurations in the Workmanship Standards and compliance to the Workmanship requirements provides excellent assurance that the hardware remain functional in missions which operate in mil-spec type environments (temperature, shock/vibe, humidity) for 15 years. This may not be the case for environment extremes (cryogenic) and very long durations (>20 years) [10].
  • Some Workmanship defects have been demonstrated to be associated with shorter service life through use and test. Some are from lessons learned feedback. Some are based on “best NASA practice” [10]. 
  • Use of Workmanship requirements criteria for non-standard, new technology may not be technically value-added or improve assembly reliability [10].

 

Workmanship requirements are better at [10]:

  • Pointing out production lines which have not “mastered” the use of a mature interconnect technology.
  • Reducing quality problems at a low level of assembly where it is less expensive to rework/repair

 

Rework vs. Repair

Rework: Process hardware to be in accordance with the drawing to correct a quality defect.

  • Existing wording in NASA standards is awkward, mentions process allowed. Improved wording in J-STD-001ES
  • Too much rework can reduce reliability. Care must be used to avoid unnecessary soldering touch-ups and part removals.
  • Rework processes must be pre-defined to ensure too much is not normally allowed.
  • Must be recorded for process engineering feedback. Rework history may be reviewed if repair is needed.

Repair: Resolve a quality defect by using a configuration that is not on the original drawing.

  • May introduce non-standard configurations and materials.
  • May introduce collateral effects such as stress on nearby interconnects or parts.
  • Must be reviewed and approved prior to use.

 

Standard vs. Non-Standard Technologies

Standard Technologies:

• Do not require special approval prior to use
• Standard Workmanship rules apply
• Examples:

  • Wire terminals (soldered to boards, wires soldered to them).
  • Surface mount solder joints: chips (0603 size and larger), gull wing.
  • Through-hole joints for DIP packages, and radial leaded and axial leaded two-connection packages.
  • Conformal coating with uralane or parylene.
  • Staking of tantalum capacitors and wire runs.
  • Using cable ties.
  • Electrical check-out of harness assembly.
  • Rosin flux and 63/39 Sn-Pb solder.
  • Mil-spec connectors: 38999 (circular), 39012 (RF), 24308 (mini-D), 83513 (micro-D).
  • Mil-spec wire and cable.
  • Wire-to-wire splicing.

Non-Standard Technologies:

• Require special approval PRIOR to use
• Standard Workmanship rules MAY NOT apply
• Examples:

  • Column Grid Array and Ball Grid Array Attachments (soon to become standard)
  • Modified commercial assemblies (COTS)
  • Pb-free solder
  • cPCI (solder tails) through-hole solder joints
  • Flex cable
  • >5 conductors in a crimp barrel
  • Cryogenic applications
  • Water soluble flux