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Navy Electricity and Electronics Training Series (NEETS)
Module 14—Introduction to Microelectronics
Chapter 3:  Pages 3-1 through 3-10

CHAPTER 3

MINIATURE AND MICROMINIATURE REPAIR PROCEDURES

LEARNING OBJECTIVES

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.

INTRODUCTION

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.
 
MINIATURE AND MICROMINIATURE ELECTRONIC REPAIR PROCEDURES
 
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.

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CAUTION
 
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
 
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:
 
·   Heat conductivity to carry heat away from components
 
·   Hardness and strength to support and protect components
 
·   Low moisture absorption
 
·   Electrical insulation
 
Conformal Coating Removal
 
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.

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Table 3-1.—Conformal Coating Removal Techniques

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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

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physically contact the workpiece surface. Delicate work handled in this manner permits you to observe the removal process.

Figure 3-2.—Hot air jet conformal coating removal.

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.

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Figure 3-3.—Rotary tool conformal coating removal.

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.
 
Conformal Coating Replacement
 
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.

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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?
 
REMOVAL AND REPLACEMENT OF DISCRETE COMPONENTS
 
To properly perform the required repair, the 2M technician must be knowledgeable of the techniques used by manufacturers in the production of electronic assemblies. The techniques, materials, and types of components determine the repair procedures used.
 
Interconnections and Assemblies
 
Assemblies may range from simple, single-sided boards with standard-sized components to double- sided or multilayered boards with miniature and microminiature components. The variations in component lead termination and mounting techniques used by manufacturers present the technician with a complex task. For example, the 2M technician is concerned about the type of solder joints on the module. To determine the solder joint type, the technician must consider the board circuitry, hole reinforcement, and lead termination style.
 
Recall the discussion from topic 1 on printed circuit board construction and the types of interconnections used. Single-sided and some double-sided boards have UNSUPPORTED HOLES where component leads are soldered to the pad. The clearance-hole method is also an interconnection with no hole support. SUPPORTED HOLES are those that have metallic reinforcement along the hole walls.
 
In addition to the plated-through hole you studied earlier, EYELETS, shown in figure 3-4, view (A), view (B), and view (C), are also used in both manufacturing and repair. These hole-reinforcing devices are usually made of pure copper, but are often plated with gold, tin, or a tin-lead alloy. The copper-based eyelet is pliable; when set, it reduces the possibility of circuit board damage. Eyelets may be inserted into single-sided or double-sided boards and are of three different types - ROLL SET, FUNNEL SET, and FLAT SET. All three are types referred to as INTERFACIAL CONNECTIONS. Interfacial connections identify the procedure of connecting circuitry on one side of a board with the circuitry on the other side.

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Figure 3-4A.—Eyelets (interfacial connections). ROLL SET

Figure 3-4B.—Eyelets (interfacial connections). FUNNEL SET

Figure 3-4C.—Eyelets (interfacial connections). FLAT SET

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As you can see, the flat-set eyelet actually provides reinforcement for the pads on both sides of the circuit board and reinforces the hole itself. The design of the roll-set eyelet (which may trap gasses, flux, or other contaminants, and obscures view of the finished solder flow) is not acceptable as a repair technique. The funnel-set eyelet does not provide as much pad reinforcement as the other types. However, it provides better "outgassing" of flux, moisture, or solvents from the space between the eyelet and the hole wall. It also provides a better view of the finished solder connection than the roll-set eyelet.
 
Lead Terminations
 
The finished circuit board consists of conductive paths, pads, and drilled holes with components and/or wires assembled directly to it. Leads and wires may terminate in three ways: (1) through the hole in the board, (2) above the surface of the board, or (3) on the surface of the board.
 
THROUGH-HOLE TERMINATION.—This style provides extra support for the circuit pads, the hole, and the lead by a continuous solder connection from one side of the circuit board to the other. Three basic variations of through-hole termination are the CLINCHED LEAD (two types), STRAIGHT- THROUGH LEAD, and OFFSET PAD.
 
Clinched Lead.—The clinched-lead termination is usually used with unsupported holes, but is found with supported holes as well. Both clinched-lead types, FULLY CLINCHED and SEMICLINCHED (figure 3-5), provide component stability. Like the fully clinched lead, the semi-clinched lead also provides stability during assembly. However, this termination can be easily straightened to allow removal of the solder joint should rework or repair be required. Note that the fully clinched lead is bent 90 degrees while the semiclinched lead is bent 45 degrees.

Figure 3-5A.—Clinched leads. FULLY CLINCHED.

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Figure 3-5B.—Clinched leads. SEMICLINCHED

Straight-Through Lead.—Straight-through terminations (figure 3-6) are used by manufacturers when the termination stability is not a prime consideration. This termination type may also be used with unsupported holes. The through-hole termination provides a better, solder-joint contact area and more solder support; the solder runs from the component side to the conductor. The straight-through termination is the easiest to remove and rework.

Figure 3-6.—Straight-through termination.

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