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.| Procedure References | |
| 1-0 | 1.0 Foreword |
| 2-1 | 2.1 Handling Electronic Assemblies |
| 2-5 | 2.5 Baking and Preheating |
Tools and Materials
Use to provide accurate measurements for thickness and hole diameters. |
Steel bars and clamps to restrain warped circuit boards during rework. |
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
Bow and Twist Repair
Check the deflection to determine if the rework is needed.
Place the restraint bars along the edges that require rework.
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.
