3.2 Bow and Twist Repair

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Outline

This procedure eliminates or reduces the bow, 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 temperatures.

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

Procedure

Check the deflection to determine if the rework is needed.
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 disposing of circuit boards with bow and twist as scrap, keep in mind how the circuit board is mounted in its final destination. Remember "form, fit, and function" without jeopardizing reliability.
Place the restraint bars along the edges that require rework. (See Figure 1)
Caution: Components or parts that interfere with the restraint bars should be removed.
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.
Place the circuit board, restraint bars, and base plate in the oven and bake for 1 hour at 125 degrees C - 257 degrees F.
Remove from the oven and allow to cool to room temperature.
Remove restraint bars.
Check the edge deflection using a caliper or pin gauges.

Evaluation

Check for marks or damage along edges.
Electrical tests as applicable.

Images and Figures

Bow and Twist Repair

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

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

Figure 3. Place the circuit board, restraint bars, and base plate in the oven and bake for 1 hour at 125 degrees C - 257 degrees F.

Figure 4. Remove from the oven and allow to cool to room temperature. Remove restraint bars. Check the edge deflection using a caliper or pin gauges.

3.2 Bow and Twist Repair

Correct bow and twist conditions in printed circuit boards using controlled heat and mechanical stabilization. Includes measurement guidelines and flattening techniques to restore dimensional stability. Minimum Skill Level: Intermediate Conformance Level: High

Correct bow and twist conditions in printed circuit boards using controlled heat and mechanical stabilization. Includes measurement guidelines and flattening techniques to restore dimensional stability.

Minimum Skill Level: Intermediate
Conformance Level: High