OUTLINE
This method is used to eliminate, or reduce the bow and twist, or warping of circuit boards. The warping is removed by controlled heating and cooling of the circuit board while under restraint.

CAUTION
This repair method is most suitable for FR-4, GE or GF substrate base materials having glass transition temperatures below 125°C (257°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

ACCEPTABILITY REFERENCES
IPC-A-600	2.0	Externally Observable Characteristics
IPC-A-610	10.0	Laminate Conditions
PROCEDURE REFERENCE
CTC 7721	1.0	Foreword
CTC 7721	2.1	Handling Electronic Assemblies
CTC 7721	2.5	Baking and Preheating
IPC 7721	32	Bow and Twist Repair

TOOLS & MATERIALS
Base Plate
Caliper or Pin Gauges
Oven
Restraint Bars
Restraint Clamps

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°C (257°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.