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3.2 Bow and Twist Repair
Outline
Procedure is used to eliminate, or reduce the bow and twist, or warping of circuit boards.

Caution
This repair method is most suitable for FR-4, GE or GF substrate base materials having glass transition temperatures below 125 degrees C (257 degrees F).

Caution
This process uses high temperatures. Some components may be sensitive to high temperature.

Note
Bow and twist should not be repaired unless sighted as a defect.

Minimum Skill Level - Intermediate
Recommended for technicians with skills in basic soldering and component rework, but may be inexperienced in general repair/rework procedures.

Conformance Level - High
This procedure most closely duplicates the physical characteristics of the original, and most probably complies with all the functional, environmental and serviceability factors.

Acceptability References
IPC-A-600 2.0 Externally Observable Characteristics
IPC-A-610 10.0 Laminate Conditions
Procedure References
1.0 Foreword
2.1 Handling Electronic Assemblies
2.5 Baking and Preheating
IPC7721 3.2 Bow and Twist Repair
Tools and Materials
Pin Gauges
Pin Gauges
Use to provide accurate measurements for thickness and hole diameters.
Oven
Restraint Bars and Clamps
Steel bars and clamps to restrain warped circuit boards during rework.
Images and Figures
3.2 Bow and Twist Repair
Figure 1: Check edge deflection for maximum warp
3.2 Bow and Twist Repair
Figure 1: Check edge deflection for maximum warp.
3.2 Bow and Twist Repair
Figure 2: Clamp restraint bars to edge needing rework.

Procedure
  1. Check the deflection to determine if rework is needed. See Figure 1.

    Note
    Bow and twist after soldering shall not exceed 1.5% for through-hole circuit boards and .75% for surface mount circuit boards. The bow and twist shall not be sufficient to cause difficulties during placement, soldering and testing operations. Before dispositioning circuit boards with bow and twist as scrap, keep in mind how the circuit board is mounted in its final destination. Keep in mind "form, fit and function" without jeopardizing reliability.  

  2. Place the restraint bars along the edges that require rework. (See Figure 2)

    Caution
    Components or parts that will interfere with the restraint bars should be removed.

  3. If the circuit board is warped along more than one edge or more than one plane, clamp the entire circuit board to the base plate.

  4. Place the circuit board, restraint bars and base plate into the oven. Bake for 1 hour at 125 degrees C - 257 degrees F.

  5. Remove from the oven and allow to cool to room temperature.

  6. Remove restraint bars.

  7. Check the edges deflection using a caliper or pin gauges.

Evaluation

  1. Check for marks or damage along edges.

  2. Electrical tests as applicable.

Procedure for reference only.