SAFETY EQUIPMENT

The nature of 2M repair requires items to be included in the tool kit for the personal safety of the technicians.  The goggles and respirator illustrated in figure 2-13 have been approved for use by the technician. These should be worn at all times where dust, chips, fumes, and other hazardous substances are generated as a result of drilling, grinding, or other repair procedures.

Figure 2-13. - Safety equipment.

STEREOSCOPIC-ZOOM MICROSCOPE

The stereoscopic-zoom microscope provides a versatile optical viewing system. This viewing system is used in the fault detection, fault isolation, and repair of complex microminiature circuit boards and components. Figure 2-14 shows the microscope mounted on an adjustable stand. The microscope has a minimum of 3.5X and a maximum of 30X magnification to detect hairline cracks in conductor runs and stress cracks in solder joints.

Figure 2-14. - Stereoscopic zoom microscope.

TOOL CHEST

The tool chest (not shown), provides storage space for the electronic repair hand tools, dental tools, abrasive wheels, solder and solder wicks, eyelets, abrasive disks, ball mills, various burrs, and other consumables used with the repair procedures.The chest is portable, lockable, and has variously sized drawers for convenience.

REPLACEMENT PARTS

Replacement parts are provided with the 2M repair set to ensure the technician has the capability to maintain the equipment properly. Actual preventive and corrective maintenance procedures, as well as data on additional spare parts and ordering information, are found in the technical manual for the 2M repair set equipment.

REPAIR STATION FACILITIES

To be effective, 2M electronic component repair must be performed under proper environmental conditions. Repair facility requirements, whether afloat or ashore, include adequate lighting, ventilation, noise considerations, work surface area, ESD (electrostatic discharge) protection, and adequate power availability. The recommended environmental conditions are discussed below. With the exception of requirements imposed by the Naval Environmental Health Center and other authorities for ship and shore work conditions, each activity tailors the requirements to meet local needs.

LIGHTING

The recommended lighting for a work surface is 100 footcandles from a direct lighting source. Light-colored overheads and bulkheads and off-white or pastel workbench tops are used to complement the lighting provided.

VENTILATION

Fumes from burning flux, coating materials, grinding dust, and cleaning solvents require adequate ventilation. The use of toxic, flammable substances, solvents, and coating compounds requires a duct system that vents gasses and vapors.This type of system must be used to prevent contamination often found in closed ventilation systems.This need is particularly important aboard ship. Vented hoods, ducts, or installations that are vented outside generally meet the minimum standards set by the Naval Environmental Health Center.

NOISE CONSIDERATIONS

Noise in the work area during normal work periods must be no greater than the acceptable level approved for each activity involved. Because the work is tedious and tiring, noise levels should be as low as possible. Ear protectors are required to be worn when a noise level exceeds 85 dB. Ear protectors should also be worn anytime the technician feels distracted by, or uncomfortable with, the noise level.

WORK SURFACE AREA.

Work stations should have a minimum work surface of at least 60-inches wide and 30-inches deep. Standard Navy desks are excellent for this purpose. Standard shipboard workbenches are acceptable; however, off-white or pastel-colored heat-resistant tops should be installed on the workbenches. Chairs should be the type with backs and without arms. They should be comfortably padded and of the proper height to match the work surface height. Drawers or other suitable tool storage areas are usually provided.

ELECTROSTATIC DISCHARGE SENSITIVE DEVICE (ESDS) CAPABILITY

A 2M work station should be capable of becoming a static-free work station. This is specified in the Department of Defense Standard, Electrostatic DISCHARGE Control Program for Protection of Electrical and Electronic Parts, Assemblies, and Equipment. ESD will be discussed in greater detail in topic 3.

POWER REQUIREMENTS.

No special power source or equipment mountinging is required. The 2M repair equipment operates on 115-volt, 60-hertz power. A 15-ampere circuit is sufficient and six individual power receptacles should be available.

HIGH-RELIABILITY SOLDERING

The most common types of miniature and microminiature repair involve the removal and replacement of circuit components. The key to these repairs is a firm knowledge of solder and high-reliability soldering techniques.

Solder is a metal alloy used to join two or more metals with a metallic bond. The bonding occurs when molten solder dissolves a small amount of the metals and then cools to form a solid connection. The solder most commonly used in electronic assemblies is an alloy of tin and lead. Tin-lead alloys are identified by their percentage in the solder; the tin content is given first. Solder marked 60/40 is an alloy of 60 percent tin and 40 percent lead. The two most common alloys used in electronics are 60/40 and 63/37.

The melting temperature of tin-lead solder varies depending on the percentage of each metal. Lead melts at a temperature of 621 degrees Fahrenheit, and tin melts at 450 degrees Fahrenheit. Combinations of the two metals melt into a liquid at different temperatures. The 63/37 combination melts into a liquid at 361 degrees Fahrenheit. At this temperature, the alloy changes from a solid directly to a liquid with no plastic or semiliquid state. An alloy with such a sharp changing point is called a EUTECTIC ALLOY.

As the percentages of tin and lead are varied, the melting temperature increases. Alloy of 60/40 melts at 370 degrees Fahrenheit, and alloy of 70/30 melts at approximately 380 degrees Fahrenheit. Alloys, other than eutectic, go through a plastic or semiliquid state in their heating and cooling stages. Solder joints that are disturbed (moved) during the plastic state will result in damaged connections. For this reason, 63/37 solder is the best alloy for electronic work. Solder with 60/40 alloy is also acceptable, but it goes into a plastic state between 361 and 370 degrees Fahrenheit. When soldering joints with 60/40 alloy, you must exercise extreme care to prevent movement of the component during cooling.

USE OF FLUX IN SOLDER BONDING.

Reliable solder connections can only be accomplished with clean surfaces. Using solvents and abrasives to clean the surfaces to be soldered is essential if you are to achieve good solder connections.In almost all cases, however, this cleaning process is insufficient because oxides form rapidly on heated metal surfaces. The rapid formation of oxides creates a nonmetallic film that prevents solder from contacting the metal. Good metal-to-metal contact must be obtained before good soldering joints may take place. Flux removes these surface oxides from metals to be soldered and keeps them removed during the soldering operation. Flux chemically breaks down surface oxides and causes the oxide film to loosen and break free from the metals being soldered.

Soldering fluxes are divided into three classifications or groups:

CHLORIDE FLUX (commonly called ACID), ORGANIC FLUX, and ROSIN FLUX. Each flux has characteristics specific to its own group. Chloride fluxes are the most active of the three groups. They are effective on all common metals except aluminum and magnesium. Chloride fluxes, however, are NOT suitable for electronic soldering because they are highly corrosive, electrically conductive, and are difficult to remove from the soldered joint.

Organic fluxes are nearly as active as chloride fluxes, yet are less corrosive and easier to remove than chloride fluxes. Also, these fluxes are NOT satisfactory for electronic soldering because they must be removed completely to prevent corrosion.

Rosin fluxes ARE ideally suited to electronic soldering because of their molecular structure. The most common flux used in electronic soldering is a solution of pure rosin dissolved in suitable solvent. This solution works well with the tin- or solder-dipped metals commonly used for wires, lugs, and connectors. While inert at normal temperatures, rosin fluxes break down and become highly active at soldering temperatures. In addition, rosin is nonconductive.

Most electronic solder, in wire form, is made with one or more cores of rosin flux. When the joint or connection is heated and the wire solder is applied to the joint (not the iron), the flux flows onto the surface of the joint and removes the oxide. This process aids the wetting action of the solder. With enough heat the solder flows and replaces the flux. Insufficient heat results in a poor connection because the solder does not replace the flux.

Q.10 Stereoscopic-zoom microscopes and precision drill presses are normally associated with what type of repair station?
Q.11 Solder used in electronic repair is normally an alloy of what two elements?
Q.12 In soldering, what alloy changes directly from a solid state to a liquid state?
Q.13 Flux aids in soldering by removing what from surfaces to be soldered?
Q.14 What type(s) of flux should never be used on electronic equipment?