Circuit Board Repair Guide > Base Board Repair > 3.2 How to Reduce the Bow and Twist, or Warping of Circuit Boards

How to Reduce the Bow and Twist, or Warping of Circuit Boards

Repair Circuit Boards | Repair SMT Pads | Repair BGA Pads | Repair Lands | Repair Edge Contacts
Repair a Conductor | Plated Hole Repair | Base Board Repair | Replace Solder Mask or Coatings

This method is used to eliminate, or reduce the bow and twist, or warping of circuit boards.

Caution
This circuit board 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 in circuit board needs repair
Bow and Twist
Note
Bow and twist should not be repaired unless sighted as a defect.

IPC Acceptability References
IPC-A-600 2.0 Externally Observable Characteristics
IPC-A-610 10.0 Laminate Conditions
 
Related Procedure References
CTC 1.0 Foreword - Circuit Board Repair Guide
CTC 2.1 Handling Electronic Assemblies
CTC 2.5 Baking and Preheating of Printed Circuit Boards
IPC7721 3.2 Bow and Twist Repair

Tools and Materials
Pin Gauges - Use to provide accurate measurements for thickness and hole diameters.
Restraint Bars and Clamps - Steel bars and clamps to restrain warped circuit boards during rework.  

3.2 Bow and Twist Repair

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

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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