Circuit Board Repair Guide > Basic Circuit Board Repair Procedures > 2.3.4 Coating Removal Using Thermal Method

Coating Removal Using Thermal Method

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This procedure for coating removal uses a controlled, low temperature, localized heating method for removing thick coatings by an overcuring or softening means.

Caution
Soldering irons should not be used for coating removal as their high operating temperatures will cause the coatings to char and possibly delaminate the printed circuit board base material.

The use of thinned down soldering iron tips or soldering iron heated thin cutting blades are not recommended since they do not provide controlled heating and may present dangerous sharp edges to the workpiece surface.

To determine the appropriate coating removal procedure the coating must first be identified. Refer to procedure number 2.3.1. Coating Removal, Identification of Coatings.
 
Coating removal required at outlined area
Coating Removal Required
At Outlined Area

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 2.2.1 How to Clean a Circuit Board
CTC 2.4.1 Replace Solder Mask or Coatings on Printed Circuit Boards
CTC 2.4.2 Replace Conformal Coatings and Encapsulants on Printed Circuit Boards
IPC 7721 2.3.4 Coating Removal, Thermal Method

Tools and Materials
Brushes
Heated Blade or Thermal Parting Tool
Hot Air Tool
Knife
Wood Sticks


Printed Board Type: R/F/W  |  Skill Level: Advanced  |  Conformance Level: High  |  Rev.: E  |  Rev. Date: Jul 7, 2000

Apply thermal tip to coating

Fig. 1: Lightly apply thermal tip to coating to soften or granulate the material.


Procedure - Thermal Parting Method
   
1.  Select an appropriate thermal parting tip to suit the workpiece configuration. Set the nominal tip temperature, using the manufacturer's recommended procedure.
 
2. Apply the thermal parting tip to the coating, using a light pressure. The coating material will either soften or granulate. Polyurethanes will soften and epoxies will granulate. The tip temperature should be regulated to a point where it will effectively "break down" the coating without scorching or charring. (See Figure 1).
 
3. Gradually reduce the coating thickness around the component body without contacting the board surface

Clip leads of component parts that are known to be faulty, thus permitting removal of the part body separately from leads and solder joints. Low pressure air or a brush should be used to remove the loosened coating.

4. Once sufficient coating has been removed, leaving only a small bonded joint between the part body and printed board, heat the component body with the thermal parting tool or hot air jet to weaken the bond beneath the component.
 
5. Lift the component body free of the printed board using small pliers.
     
Note
Twist the component prior to removal to shear any remaining epoxy bond to the printed board surface.

6. Once the component body has been removed from the board surface, the remaining coating material can be removed by additional thermal parting. The remaining leads and solder joints are then removed by appropriate solder extraction means.
 
Apply hot air to remove coating

Fig. 2: Apply hot air to the work
area and use a wood stick to remove the overcured coating.
Procedure - Hot Air Method
   
By control of the gas/air temperature, flow rates and jet shape, the hot air method can be applied to almost any workpiece configuration on both the component and solder side of the printed board without damage. Extremely delicate work can be handled in this manner while permitting direct observation of the heating action.
 
1.  Set up the hot air tool per the manufacturer's instructions. Adjust flow rate and temperature to suit specific coating removal application.
     
Caution
Never set the gas/air temperature at a level that will cause scorching or charring of the coating material or reflow the solder connections.
   
Coating removal complete

Fig. 3: Coating removal complete.
2. Apply the heated air jet to work area. Apply light pressure using a wood stick or other non marring tool to remove the softened or overcured coating. All coating around individual leads, solder joints and component bodies can be removed in this manner. (See Fig. 2).
 
3. When the coating has been removed, use appropriate solder extraction method to remove components if needed
   
Evaluation
   
1.  Visual examination or UV light may be used to verify complete removal of  coating.




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