Upon completion of this topic, the student will be able to:
1. Explain the purpose of conformal coatings and the methods used for removal and replacement of these coatings.
2. Explain the methods and practices for the removal and replacement of discrete components on printed circuit boards.
3. Identify types of damage to printed circuit boards, and describe the repair procedures for each type of repair.
4. Describe the removal and replacement of the dual-in-line integrated circuit.
5. Describe the removal and replacement of the TO-5 integrated circuit.
6. Describe the removal and replacement of the flat-pack integrated circuit.
7. Describe the types of damage to which many microelectronic components are susceptible and methods of preventing damage.
8. Explain safety precautions as they relate to 2M repair.
As you progress in your training as a technician, you will find that the skill and knowledge levels required to maintain electronic systems become more demanding. The increased use of miniature and microminiature electronic circuits, circuit complexity, and new manufacturing techniques will make your job more challenging. To maintain and repair equipment effectively, you will have to duplicate with limited facilities what was accomplished in the factory with extensive facilities. Printed circuit boards that were manufactured completely by machine will have to be repaired by hand.
To meet the needs for repairing the full range of electronic equipment, you must be properly trained. You must be capable of performing high-quality, reliable repairs to the latest circuitry.
As mentioned at the beginning of topic 2, 2M repair personnel must undergo specialized training. They are trained for a particular level of repair and must be certified at that level. Also, recertification is required to ensure the continued high-quality repair ability of these technicians.
THIS SECTION IS NOT, IN ANY WAY, TO BE USED BY YOU AS AUTHORIZATION TO ATTEMPT THESE TYPES OF REPAIRS WITHOUT OFFICIAL 2M CERTIFICATION.
In the following sections, you will study the general procedures used in the repair, removal, and replacement of specific types of electronic components. By studying these procedures, you will become familiar with some of the more common types of repair work. Before repair work can be performed on a miniature or microminiature assembly, the technician must consider the type of specialized coating that usually covers the assembly. These coatings are referred to as CONFORMAL COATINGS.
Conformal coatings are protective material applied to electronic assemblies to prevent damage from corrosion, moisture, and stress. These coatings include epoxy, parylene, silicone, polyurethane, varnish, and lacquer. Coatings are applied in a liquid form; when dry, they exhibit characteristics that improve reliability. These characteristics are:
Because of the characteristics that conformal coatings exhibit, they must be removed before any work can be done on printed circuit boards. The coating must be removed from all lead and pad/eyelet areas of the component. It should also be removed to or below the widest point of the component body. Complete removal of the coating from the board is not done.
Methods of coating removal are thermal, mechanical, and chemical. The method of removal depends on the type of coating used. Table 3-1 shows suggested methods of removal of some types. Note that most of the methods are variations of mechanical removal.
The coating material can best be identified through proper documentation; for example, technical manuals and engineering drawings. If this information is not available, the experienced technician can usually determine the type of material by testing the, hardness, transparency, thickness, and solvent solubility of the coating. The thermal (heat) properties may also be tested to determine the ease of removal of the coating by heat. The methods of removal discussed here describe the basic concept, but not the step-by-step "how to" procedures.
THERMAL REMOVAL.—Thermal removal consists of using controlled heat through specially shaped tips attached to a handpiece. Soldering irons should never be used for coating removal because the high temperatures will cause the coatings to char, possibly damaging the board materials. Modified tips or cutting blades heated by soldering irons also are not used; they may not have proper heat capacity or allow the hand control necessary for effective removal. Also, the thin plating of the circuit may be damaged by scraping.
The thermal parting tool, used with the variable power supply, has interchangeable tips, as shown in figure 3-1, that allow for efficient coating removal. These thin, blade-like instruments act as heat generators and will maintain the heat levels necessary to accomplish the work. Tips can be changed easily to suit the configuration of the workpiece. These tips cool quickly after removal of power because their small thermal mass and special alloy material easily give up residual heat.
Figure 3-1.—Thermal parting tips.
The softening or breakdown point of different coatings vary, which is a concern when you are using this method. Ideally, the softening, point is below the solder melting temperature. However, when the softening point is equal to or above the solder melting point, you must take care in applying heat at the solder joint or in component areas. The work must be performed rapidly to limit the heating of the area involved and to prevent damage to the board and other components.
HOT-AIR JET REMOVAL.—In principle, the hot-air jet method of coating removal uses controlled, temperature-regulated air to soften or break down the coating, as shown in figure 3-2. By controlling the temperature, flow rate, and shape of the jet, you may remove coatings from almost any workpiece configuration without causing any damage. When you use the hot-air jet, you do not allow it to physically contact the workpiece surface. Delicate work handled in this manner permits you to observe the removal process.
POWER-TOOL REMOVAL DESCRIPTION.—Power-tool removal is the use of abrasive grinding or cutting to mechanically remove coatings. Abrasive grinding/rubbing techniques are effective on thin coatings (less than 0.025 inch) while abrasive cutting methods are effective on coatings greater than 0.025 inch. This method permits consistent and precise removal of coatings without mechanical damage or dangerous heating to electronic components. A variable-speed mechanical drive handpiece permits fingertip-control and proper speed and torque to ease the handling of gum-type coatings. A variety of rotary abrasive materials and cutting tools is required for removal of the various coating types. These specially designed tools include BALL MILLS, BURRS, and ROTARY BRUSHES.
The ball mill design places the most efficient cutting area on the side of the ball rather than at the end. Different mill sizes are used to enter small areas where thick coatings need to be removed (ROUTED). Rubberized abrasives of the proper grade and grit are ideally suited for removing thin, hard coatings from flat surfaces; soft coatings adhere to and coat the abrasive causing it to become ineffective. Rotary bristle brushes work better than rubberized abrasives on contoured or irregular surfaces, such as soldered connections, because the bristles conform to surface irregularities. Ball mill routing and abrasion removal are shown in figure 3-3.
CUT AND PEEL.—Silicone coatings (also referred to as RTV) can easily be removed by cutting and peeling. As with all mechanical removal methods, care must be taken to prevent damage to either components or boards.
CHEMICAL REMOVAL.—Chemical removal uses solvents to break down the coatings. General application is not recommended as the solvent may cause damage to the boards by dissolving the adhesive materials that bond the circuits to the boards. These solvents may also dissolve the POTTING COMPOUNDS (insulating material that completely seals a component or assembly) used on other parts or assemblies. Only thin acrylic coatings (less than 0.025 inch) are readily removable by solvents. Mild solvents, such as ISOPROPYL ALCOHOL, XYLENE, or TRICHLOROETHANE, may be used to remove soluble coatings on a spot basis.
Evaluations show that many tool and technique combinations have proven to be reliable and effective in coating removal; no single method is the best in all situations. When the technician is determining the best method of coating removal to use, the first consideration is the effect that it will have on the equipment.
Once the required repairs have been completed the conformal coating must be replaced. To ensure the same protective characteristics, you should use the same type of replacement coating as that removed.
Conformal coating application techniques vary widely. These techniques depend on material type, required thickness of application, and the effect of environmental conditions on curing. These procedures cannot be effectively discussed here.
Q1. What material is applied to electronic assemblies to prevent damage from corrosion, moisture, and stress?
Q2. What three methods are used to remove protective material? Q3. What chemicals are used to remove protective material?
Q4. Abrasion, cutting, and peeling are examples of what type of protective material removal? Q5. Why should the coating material be replaced once the required repair has been completed?
Labels: Introduction to Microelectronics
