MIL-PRF-39015 Resistors, Variable, Wirewound, \(Lead Screw Actuated\), Nonestablished Reliability, And Established Reliability, General Specification ForThis specification covers the general requirements for nonestablished reliability \(non-ER\) and established reliability \(ER\), lead screw actuated; wirewound variable resistors with a contact bearing uniformly over the surface of the entire resistive element, wound linearly, when positioned by a multiturn lead screw actuator. These resistors are capable of full load operation \(when the maximum resistance is engaged\), at a maximum temperature of 85OC and are capable for continuous operation, when properly derated, at maximum temperature of 150OC \(see 3.6\), with a resistance tolerance of ??5 percent. The ER resistors have life failure rate \(FR\) levels ranging from 1.0 percent to 0.001 percent per 1,000 hours \(see 1.2.1.4\). These FR levels are established at a 60 percent confidence level on the basis of life tests.

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MIL-PRF-39015E

6.8 Reduction of power rating. When only a portion of the resistance element is engaged, the wattage

is reduced in directly the same proportion as the resistance.

6.9 Stacking of resistors. When stacking of resistors, care should be taken to compensate for the

added rise in temperature by derating the wattage rating accordingly.

6.10 MIL-PRF-27208 substitution data. Resistors of this specification, regardless of their FR

designation, are substitutes for resistors of the same resistance value, tolerance, and performance

characteristics specified in the inactive for new design specification of MIL-PRF-27208 as follows:

Substitute

Specification inactive

specification

for new design

Associated specification information is provided in the applicable specification sheet.

6.11 Lead material. Weldable lead material type N-2 of MIL-STD-1276 has been deleted as a

requirement from MIL-PRF-39015; the user's attention is directed to this change. Parts acquired to this

specification are interchangeable with parts to all previous issues, when the application required a

solderable lead. If weldable leads with specific materials are required, such requirement must be

specified in the contract or order.

6.12 Retinning leads. If retinning (hot solder dip) of the leads is required, see 3.5.8.

6.13 Tin whisker growth. The use of alloys with tin content greater than 97 percent, by mass, may

exhibit tin whisker growth problems after manufacture. Tin whiskers may occur anytime from a day to

years after manufacture and can develop under typical operating conditions, on products that use such

materials. Conformal coatings applied over top of a whisker-prone surface will not prevent the formation

of tin whiskers. Alloys of 3 percent lead, by mass, have shown to inhibit the growth of tin whiskers. For

additional information on this matter, refer to ASTM-B545 (Standard Specification for Electrodeposited

Coatings of Tin).

6.14 Caution note. The QPL manufacturer of the "L" termination of MIL-PRF-39015/2 has always used

100 percent tin plating on their internal tabs. Termination "L" is inactive for new design, and designers

are cautioned on using the "L" termination for replacement purposes (see 6.13).