Coating removal methods: normally, removing a large QFF from a board would be easy, but what do you do with thick, clear, rubbery coatings?

Not all coatings are alike--different materials exhibit different physical properties. Some coatings can be dissolved with solvent; some cannot. Some coatings can be removed after softening through heating; for others, heating is ineffectual.

Determining Coating Type

The rework technician, or his or her supervisor, must often determine what type of coating material is being used--materials range from acrylics to epoxies. The rework technician must first identify the coating to understand what removal method should be applied.

If the technician does not know the specific identity of a coating prior to removal, a simple procedure exists to help determine the coating material. The procedure uses simple observation and testing to help identify coating characteristics. Testing incorporates coating hardness, transparency, solubility, thermal properties, stripp ability and thickness.

Once the type of coating is determined, the technician chooses the proper removal method. More than one method may be used, per type of coating.

Determining Removal Method

Coating hardness is one criteria used to determine the removal method used. The technician can perform a penetration test, in a non-critical area, to determine relative hardness. Purely abrasive removal techniques are more suited to hard coatings. Solvent removal procedures are more suited to softer coatings. Transparency is also a factor in determining the removal technique. Transparent coatings are usually better suited for removal than opaque coatings. In addition, the technician may test the coating for solubility characteristics in a non-critical area with trichloroethane, xylene or other solvent with low toxicity and mild activity.

To determine suitability for thermal removal, the technician must use a thermal parting device, with controlled heating and without a cutting edge, to determine the possibility of thermal removal. Start with a low temperature, approximately 100[degrees]C, and increase the temperature until the coating is removed. If the coating flows or gums up, the temperature is too hot or the coating is not suitable for thermal removal.

A technician may determine strippability by carefully slitting the coating with a sharp blade, in a noncritical area, and trying to peel the coating back from the surface. Due to the adhesion required of coating materials, strippable techniques--without chemical aids--are usually very limited.

Thick coatings usually require a two-step removal method to prevent surface damage to the board. The technician first reduces the thickness of the coating and then uses pure abrasion methods to reach the surface of the board.

The techniques employed in coating removal are well documented and require skill and practice to perform. Each technique is too involved to describe in this brief column, but here are a few pointers:

* Tools required for coating removal vary widely. A blunt knife or blade is needed for peeling--a tool that can easily be customized.

* Spot removal with solvents usually involves nothing more complicated than swabs dipped in solvent and rubbing action.

* Low-pressure air jets may be used to get under lifted coating edges.

* Beware of static generation in cases using grinding or abrasion.

* In thermal removal applications, polyurethanes, when heated, will soften and epoxies will granulate. Never set the gas/air temperature at a level that will cause scorching or charring of the coating material or reflow of the solder connections.

* The grinding/scraping method uses various tools depending on the composition of the coating material--a knife or dental style scraper or handheld drill may be used. A wide variety of rotary abrasive materials, including ball mills, may be required.

* The micro-blasting method uses a micro-abrasive blasting system and a fine, soft, abrasive powder that is propelled through a small nozzle.

Coating Identification and Removal Method Tables

These tables are referenced in Jeff Ferry's Rework and Repair Depot column. "Coating Removal Methods," (October 2002, pg 56).

Table 1 is a quick and reliable guide to determine coating types, in the first line, for example, depending on the answer, "yes" or "no," the technician proceeds to the numbered question indicated in the column.

TABLE 1: Coating identification.

Test                                      Yes              NO

1. Does the coating feel soft, rubbery
   or spongy?                             2                 3
2. Does the coating have a noticeable
   reaction to heat?                      Polyurethane      4
3. Is there a reaction to alcohol?        Acrylic           5
4. Is the coating thick and does
   it have a dull surface?                Silicone Thick   Silicone Thin
5. Does the coating have a noticeable
   reaction to heat?                      6                Paraxylyene
6. Does the reaction form white powder?   Epoxy            Polyurethane

Table 2 is a guide, but the technician should take into account the overall characteristics of the coating, as these will indicate the best choice of removal procedure.

TABLE 2: Conformal coating removal methods.

Conformal Coating   Removal Method

                    2.3.2     2.3.3     2.3.4     2.3.5      2.3.6
                    Solvent   Peeling   Thermal   Grinding   Micro
                    Method    Method    Method    Scraping   Blasting
                                                  Method     Method

Paraxylyene                               1         2          3
Epoxy                                     1         2          3
Acrylic                1                  2         3          4
Polyurethane           3                  1         2          4
Silicone, Thin         1                  2         3          4
Silicone, Thick                 1                   2

Jeff Ferry is president of Circuit Technology Center, Haverhill, MA; [phone]; www.circuittechctr.com