Mil-std-1250 pdf

Materials not inherently moisture and fungus-resistant q. RA solder flux r. Gold over silver or copper without a barrier coat of nickel s. Organic materials that outgas corrosive vapors t. Polymide insulation.

All surfaces shall be thoroughly cleaned before joining, coating, potting, impregnating, or encapsulating. Surfaces shall be cleaned after brazing, soldering or welding to remove contaminants. High strength steels over ksi or HRC 40 shall not be cathodically cleaned in either acid or alkaline baths and shall not be acid cleaned except anodically. Organic solvents for use on plastics shall be selected with care to avoid crazing or other damage to the plastic. For vapor degreasing, solvent cleaning and drying, only those materials and procedures listed in MIL-STD shall be used.

Care shall be taken that cleaning materials are compatible with all materials in the part or assembly. Dust, dirt and fingerprints shall be removed prior to assembly or storage. Printed wiring assemblies PWAs shall be cleaned and tested prior to conformal coating.

The PWA cleanliness should be checked by rinse solution resistivity measurement. Refer to MIL-P for cleaning and testing requirements. Printed circuit board laminates shall not be roughened by over-zealous cleaning to remove solder flux. Attention shall be given to protection of items from dust and contaminants during manufacture.

Handling shall be restricted to a minimum, fingerprints shall be removed before assembly, and protective bags shall be used during in-process storage where needed. The design and construction shall minimize the corrosive effect of oxygen, moisture and airborne corrosives in the atmosphere within electronic equipment.

Techniques to be used, as applicable, for prevention of deterioration include protective coating or encapsulation of components, evacuation and hermetic seals, filtering air and removal of moisture. Materials which break down or outgas shall not be incorporated in components or devices which are evacuated and sealed. Partially cured or undercured organic materials shall be avoided.

For maximum protection against environments, components shall be evacuated and hermetically sealed by fusion of metals, glass or ceramics. Consideration shall be given to possible corrosion from condensation of moisture unless hermetically sealed units are evacuated before sealing, and to contamination of contact surfaces by outgassing materials.

Precautions shall be taken to remove moisture, dust and contaminants from air which is inside, or which enters, compartments of electronic devices. When forced air cooling is necessary to maintain equipment at temperatures below the maximum permissible operating temperature, moisture and contaminants shall be removed, preferably externally, before cooling air passes over electronic components.

Unless otherwise specified, the moisture level inside electronic equipment shall be maintained below 45 percent relative humidity at 20 Celsius C. Moisture shall be excluded from electronic devices by adequate housings, seals, gaskets and closures.

The following measures shall be taken:. Avoid pockets, wells, traps and sump areas where water and condensed moisture can collect, or provide drainage paths b. Avoid hygroscopic materials c. Desiccants shall not be used unless necessary for moisture level control, and then only where adequate surveillance schedules have been established for each specific application.

In no case shall desiccant material be in contact with unprotected metallic parts. Unless otherwise specified, desiccants shall be in accordance with MIL-D d. Preclude condensation by keeping components at temperature above the dewpoint.

Metals shall be selected which are suitable for the purpose and which are inherently resistant to corrosion or made corrosion resistant by coating or plating. Dissimilar metals shall not be used in contact or close proximity unless suitably. Special attention shall be given to the following problem areas, which can result in lower conductivity, "noise," short circuits or broken leads. Where maximum conductivity is required on items exposed to the atmosphere as on contacts , metals which are inherently resistant to oxidation and tarnish shall be selected, such as gold, rhodium and platinum.

When other metals are used, the surface shall be protected from oxidation by plating with the noble metals listed above or by suitable noninsulating coatings. Preference shall be given to metals which are resistant to both intergranular and stress corrosion, especially for applications involving residual and induced stresses.

The alloys which are normally most resistant to intergranular corrosion are also more resistant to stress corrosion.

Preference shall be given these alloys, especially for applications involving residual and induced stresses. In addition, all bending, forming, and shaping shall be performed on metal in the annealed condition.

Every effort shall be made to use the lowest stress level practical. Preference shall be given to metals which are not susceptible to delayed fracture due to hydrogen pickup from acid cleaning or plating, such as the series corrosion resistant steels and oxygen-free copper. Where it is necessary to use metals which are susceptible to hydrogen pickup, coating methods shall be selected in accordance with restrictions in MIL-S In addition, the following methods shall be used to minimize damage:.

Organic coating, vapor deposition, mechanical plating, metal spraying and other non-hydrogen-producing processes shall be used in preference to electroplating or chemical plating b. If plating is necessary, low-hydrogen-embrittlement baths shall be used c. Parts shall be embrittlement- relieved immediately after plating in accordance with the appropriate specification d. Where practical, parts shall be thermally stress relieved prior to plating, for a minimum of three hours at 14C e. Where practical, parts shall be mechanically stress relieved prior to plating, by shot peening in accordance with MIL-S f.

Neither acid nor alkaline cathodic cleaning shall be used on metals susceptible to hydrogen embrittlement. The growth of metal whiskers on tin, cadmium, silver, or iron shall be minimized by the following techniques:.

Use of heavy, rather than thin, metal coatings b. Use of hot dip tin rather than electrodeposited tin c. Reheating tin plating to relive stresses d. Maintaining a low level of humidity in the equipment e. Use of tin plating with 2 to 5 percent co-deposited lead.

The movement of metal from one conductor in a circuit to another which is at a different voltage potential under humid conditions, across ceramic or plastic insulation, is especially liable to occur when one of the metals is silver. This phenomenon of electrolysis or silver migration shall be minimized by use of the following methods, as applicable:. The spacing between conductors at different voltage potentials shall be as wide as possible; b.

Conductors shall be protected by an organic moisture barrier coating such as MIL-I c. Contaminants shall be removed from conductor surface by careful cleaning followed by rinsing with deionized water, and thorough drying prior to conformal coating; d. Where practicable, gold, platinum, or tin-lead coatings shall be used in lieu of silver; e. Humidity shall be maintained at a level which precludes condensation under anticipated environments; f.

Nonhygroscopic insulation shall be used. Metallic coatings shall be selected for their suitability for the application involved. Attention shall be given to problems of aging, cracking, diffusion and corrosion. When metallic coatings are applied by electroplating, special care shall be taken to avoid hydrogen embrittlement.

In selecting metallic coatings, the recommendations shown in table I for preventing corrosion shall be given consideration. Cadmium and cadmium-plated parts are susceptible to attack by corrosive vapors. Cadmium shall not be used in enclosed assemblies with materials such as acid, ammonia, adhesives, coatings, plastics, varnishes or other organic materials which give off corrosive vapors. Cadmium plating shall not be used in space applications. Chromium plating shall be in accordance with QQ-C It shall be used for applications involving wear or requiring hardness, and not for corrosion prevention.

Copper oxide corrosion products Red Plague which occur at pinholes or porosity in other metallic plating over copper, may be precluded by the interposition of a layer of nickel between the copper and the top metallic coating.

Prepaint coating Cadmium or tin Chromium, copper, nickel, gold, silver. Gold over silver, copper or nickel. Nickel between copper and silver See 4.

Marine exposure Heavy gold 0. Solderability Tin, gold or tin-lead Nickel, chromium, rhodium. Storage Gold, rhodium, or reflowed Cadmium, silver, copper heavy tin. Wear Chromium, nickel, Cadmium, tin rhodium or hard gold. Gold plating shall be in accordance with MIL-G Special care shall be taken to prevent or retard diffusion of substrate metals, especially copper into the gold electro-deposits.

A suitable barrier to prevent diffusion is a thin nickel or palladium coating under the gold. Thickness of gold shall be sufficient to minimize porosity and provide corrosion protection. The recommended minimum thickness of gold plating is 0. Attention is directed to problems of brittle gold solder compound formation see 5. Corrosion resistance shall meet the requirement of AMS Where desired for electrical bonding, nickel-cadmium coating shall be in accordance with AMS Nickel shall be.

Electrical connectors with finishes of nickel plating shall not be used. Palladium plating shall be in accordance with MIL-P- The use of palladium plating in enclosed assemblies containing organic materials shall be avoided, to prevent polymerization of organic compounds. Silver plating shall be in accordance with QQ-S Silver plating may be passivated for temporary protection, or may be over-coated with rhodium in accordance with MIL-R to prevent tarnish.

Silver plating shall be protected from sulfurous fumes during storage, and shall be cleaned immediately prior to soldering. Attention shall be given to prevention of silver migration see 5. Silver plating shall not be used for contacts without prior approval from the procuring activity. Tin coating shall be applied by hot dip or electrodeposition in accordance with MIL-T For maximum protection, tin shall be reflowed diffused after coating.

Where desired for electrical bonding, tin-cadmium plating shall be in accordance with MIL-P Care shall be taken to prevent formation of metal whiskers on tin surfaces during storage see 5. Zinc plating may be used as an alternate coating for cadmium where it meets the requirements for the component or system.

However, cadmium plating is the preferred choice for avionics and systems used in marine environments. Zinc plating shall not be used without prior approval from the procuring activity. Metallic coatings may be applied to nonmetallics to provide a conductive surface.

Although problems of dissimilar metal corrosion are thereby minimized or eliminated, other corrosion reactions of the metal coating shall be considered in the same manner as for plated or solid metals. Metallic coatings may be applied by vapor deposition to either metallic or nonmetallic surfaces for electrical conductivity.

Vapor deposited coatings shall not be used for any mechanical application because of their extreme thinness, fragility and susceptibility to damage. Aluminum coatings shall be applied in accordance with MIL-C Surface porosity in castings shall be impregnated in accordance with MIL-STD before receiving appropriate surface finish. Impregnants for aluminum castings shall be in accordance with MIL-I Corrosion resistant steels shall be passivated in accordance with QQ-P No further finish is required to provide corrosion resistance to steels of the series.

Where tarnish, rust or surface stain is objectionable, the series and precipitation hardening steels shall be given additional protection by a suitable plating or paint finish after passivation.

Ordinary iron and steel shall be coated with cadmium, nickel or tin except as follows:. Precision parts which are totally and continuously immersed in oil, grease, encapsulant or moisture-proof coating, or which are contained in hermetically sealed units, need not be given any further protection.

Local application of oil or grease shall not automatically be considered corrosion protection. Laminations used in magnetic circuits need not be plated if they are otherwise protected from corrosion c.

Springs shall preferably be given organic coatings, or be coated by ion deposition or other non-hydrogen-producing processes, rather than electroplated d. The noble metals gold, palladium, platinum and rhodium and the corrosion-resistant metals chromium, nickel, and titanium require no finish other than cleaning.

Surface finish for electrical bonding or grounding is listed in 5. Applications of aluminum and copper shall receive the protection specified in 5. In areas of electrical bonding see 5. For applications where anodizing is not required, chemical film treatment in accordance with MIL-C may be used, although it is not to be considered to afford corrosion protection equivalent to anodize.

Whenever possible, chemical films shall be given the additional protection of organic coatings. For applications involving continued exposure to elevated temperatures, where the resultant oxide buildup is objectionable such as heat sinks consideration shall be given to metallic coatings with suitable thermal characteristics in lieu of anodize.

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