IPC 7711A.pdf - 第214页
3. If the printed wiring board is warped along more than one edge or more than one plane, clamp the entire printed wiring board to the base plate. 4. Place the printed wiring board, restraint bars and base plate into the…
OUTLINE
This method is used to eliminate, or reduce the bow and twist, or warping of printed
wiring boards. The warping is removed by controlled heating and cooling of the
printed wiring 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. The bake/time cycle will have to
be adjusted depending on the base material glass transistion temperature.
CAUTION
This process uses high temperatures. Some components may be sensitive to high
temperature and should be removed if this procedure will adversely affect them.
CAUTION
High temperatures will cause oxidation of solderable surfaces.
NOTE
Bow and twist should not be repaired unless sited as a defect.
REFERENCES
2.1 Handling Electronic Assemblies
2.5 Baking and Preheating
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 printed wir-
ing boards and 0.75% for surface mount printed wiring boards. The bow and
twist shall not be sufficient to cause difficulties during placement, soldering and
testing operations. Before dispositioning printed wiring boards with bow and twist
as scrap, keep in mind how the printed wiring board is mounted in it’s final des-
tination. 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.
Figure 1 Check edge deflection for
maximum wrap.
Figure 2 Clamp restraint bars to edge
needing rework.
7721A
Repair and
Modification of
Printed Boards and
Electronic Assemblies
Revision:
Date: 2/98
Bow and Twist Repair
Number: 3.2
Product Class: R, W
Skill Level: Advanced
Level of Conformance: Medium
Material in this manual was voluntarily established by Technical Committees of IPC. This material is advisory only and its use
or adaptation is entirely voluntary. IPC disclaims all liability of any kind as to the use, application, or adaptation of this material.
Users are also wholly responsible for protecting themselves against all claims or liabilities for patent infringement. Equipment
referenced is for the convenience of the user and does not imply endorsement by IPC.
Page1of2
Copyright Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
--``,``,-`-`,,`,,`,`,,`---
3. If the printed wiring board is warped along more than one edge or more than one
plane, clamp the entire printed wiring board to the base plate.
4. Place the printed wiring board, restraint bars and base plate into the oven. Bake
for 1 hour at 125°C.
NOTE
If possible, after the 1 hour bake cycle, shut off the oven and leave the printed
wiring board inside. This will allow the printed wiring board to slowly cool to room
temperature improving stress relief.
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.
NOTES
IPC-7721A
Number: 3.2
Revision:
Date: 2/98
Subject: Bow and Twist Repair
Page2of2
Copyright Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
--``,``,-`-`,,`,,`,`,,`---
OUTLINE
This method is used to repair cosmetic defects or minor damage to an unsupported
tooling or mounting hole. The hole may have component leads, wires, fasteners,
pins, terminals or other hardware run through it. This repair method uses high
strength epoxy to restore the damaged surface surrounding the hole. This method
can be used on single sided, double sided or multilayer printed wiring boards and
assemblies.
CAUTION
Damaged inner-layer connections may require surface wire adds.
REFERENCES
2.1 Handling Electronic Assemblies
2.2 Cleaning
2.5 Baking and Preheating
2.6 Epoxy Mixing and Handling
TOOLS & MATERIALS
Ball Mills
Cleaner
Color Agent, Various Colors
Epoxy
Hand Held Drill
Polyimide Tape
Knife
Mixing Sticks
Oven
Wipes
PROCEDURE
1. Clean the area.
2. Mill away the damaged board base material using the hand held drill and ball
mill. All damaged base board material and solder resist must be removed. No
fibers of laminate material should be exposed at the surface peremiter of the
hole. (See Figure 1.)
NOTE
To clearly see that all damaged material has been removed, flood the area with
alcohol or solvent. Damaged internal fibers of the base material will show up
clearly.
3. Remove all loose material and clean the area.
4. Where needed, apply Polyimide tape to protect exposed parts of the printed
wiring board. Tape may be required inside the hole. If epoxy reduces the inside
diameter, the hole may have to be redrilled after the epoxy has cured.
NOTE
The printed wiring board may be preheated prior to filling the area with epoxy.
A preheated printed wiring board will allow the epoxy to easily flow and level out.
Figure 1 Mill away damaged material.
Figure 2 Apply epoxy with a small
wood stick sharpened at one end.
7721A
Repair and
Modification of
Printed Boards and
Electronic Assemblies
Revision:
Date: 2/98
Hole Repair,
Epoxy Method
Number: 3.3.1
Product Class: R, W
Skill Level: Advanced
Level of Conformance: High
Material in this manual was voluntarily established by Technical Committees of IPC. This material is advisory only and its use
or adaptation is entirely voluntary. IPC disclaims all liability of any kind as to the use, application, or adaptation of this material.
Users are also wholly responsible for protecting themselves against all claims or liabilities for patent infringement. Equipment
referenced is for the convenience of the user and does not imply endorsement by IPC.
Page1of2
Copyright Association Connecting Electronics Industries
Provided by IHS under license with IPC
Not for Resale
No reproduction or networking permitted without license from IHS
--``,``,-`-`,,`,,`,`,,`---