Circuit Board Repair and Rework Guide > 3.0 Base Board Procedures > 3.2 Bow and Twist Repair Bookmark and Share

3.2 Bow and Twist Repair

Printed Board Type: R/W  |  Skill Level: Intermediate  |  Conformance Level: Medium  |  Rev.: D  |  Rev. Date: Jul 7, 2000

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° C (257° F).

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

   Bow and Twist
Bow and Twist
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 References
CTC 1.0 Foreword
CTC 2.1 Handling Electronic Assemblies
CTC 2.5 Baking and Preheating
IPC7721 3.2 Bow and Twist Repair

Tools and Materials
Base Plate
Caliper or Pin Gauges
Oven
Restraint Bars
Restraint Clamps  

Procedure

Check edge deflection for maximum warp.

Figure 1: Check edge deflection for maximum warp.
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.
   
Clamp restraint bars to edge needing rework.

Figure 2: Clamp restraint bars to
edge needing rework.
2.  Place the restraint bars along the edges that require rework. (See Figure 2).
 
 
Note
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.


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3.2 Bow and Twist Repair