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2.3.4 Coating Removal, Thermal Method
Outline
This coating removal procedure 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 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.

Minimum Skill Level - Advanced
Recommended for technicians with soldering and component rework skills and exposure to most repair/rework procedures, but lacking extensive experience.

Conformance Level - High
This procedure most closely duplicates the physical characteristics of the original, and most probably complies with all the functional, environmental and serviceability factors.

Acceptability References
IPC-A-600 2.0 Externally Observable Characteristics
IPC-A-610 10.0 Laminate Conditions
Procedure References
1.0 Foreword
2.1 Handling Electronic Assemblies
2.2 Cleaning
2.3.1 Coating Removal, Identification Of Coatings
2.4.1 Coating Replacement, Solder Mask
2.4.2 Coating Replacement, Conformal Coatings/Encapsulants
IPC7721 2.3.4 Coating Removal, Thermal Method
Tools and Materials
CircuitMedic Part
Cleaning Brushes
Disposable brushes for use with solvents for cleaning and application of coatings.
Hot Air Tool
Hot Air Tool
Tool and nozzles to deliver precise flow of hot air.
CircuitMedic Part
Knife with #16 Blades
A must-have tool for precise cutting, scraping and trimming.
CircuitMedic Part
Mixing Picks
Unique mixing sticks have a paddle shape on one and sharp pick on the opposite end.
Thermal Part Tool
Thermal Parting Tool
Thermal removal tool using controlled heat through specially shaped tips.
Images and Figures
2.3.4 Coating Removal, Thermal Method
Coating Removal Required At Outlined Area
2.3.4 Coating Removal, Thermal Method
Figure 1: Lightly apply thermal tip to coating to soften or granulate the material.
2.3.4 Coating Removal, Thermal Method
Figure 2: Apply hot air to the work area and use a wood stick to remove the overcured coating.
2.3.4 Coating Removal, Thermal Method
Figure 3: Removal complete.

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.

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.

  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 Figure 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.

Procedure for reference only.