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

3.3.1 SPC system. As part of the overall MIL-STD-790 QPL system, the manufacturer shall establish a

SPC system that meets the requirements of EIA-557. Typical manufacturing processes for application of

SPC include lead attachment, weld strength, and winding or wire. The reliability of resistors furnished

under this specification shall be established and maintained in accordance with the requirements and

procedures specified in MIL-STD-790 and MIL-STD-690 with details and exceptions specified in 4.2,

3.3.2 PPM system. As part of the overall MIL-STD-790 QPL system, the manufacturer shall establish

a PPM system for assessing the average outgoing quality of lots in accordance with EIA-554-1 and 4.6.4.

Data exclusion, in accordance with EIA-554-1, may be used with approval of the qualifying activity. The

PPM system shall identify the PPM rate at the end of each month and shall be based on a six month

moving average. PPM and total resistance shall be assessed for each style. Style reporting may include

both non-ER and ER style combinations.

3.4 Material. The material shall be as specified herein. However, when a definite material is not

specified, a material shall be used which will enable the resistors to meet the performance requirements

of this specification. Acceptance or approval of any constituent material shall not be construed as a

guarantee of the acceptance of the finished product.

3.4.1 Plastic. Plastic laminates containing a cotton-fabric base or plastic-molding compounds

containing cotton or wood-flour filler shall not be used. Where not machined, plastic material shall have

there original, smooth, or polished surfaces. Surfaces that have been sawed, cut, punched, or otherwise

machined shall be smooth as practicable in accordance with good manufacturing practice for the

intended application.

3.4.2 Ferrous materials. Unless specifically approved by the Government, the use of ferrous material,

with the exception of corrosion-resistant steel and resistance-element material is prohibited.

3.5 Interface and physical dimensions requirements. Resistors shall meet the interface, and physical

dimensions specified (see 3.1).

3.5.1 Resistance element. The resistance element shall be wound on a form which shall not char or

break down as a result of the tests specified herein. The element shall be a continuous unbroken length

of conductor without joints, bonds, or welds, except at the junction of the resistor element and the winding

terminals. The wire shall possess a uniform cross-sectional area and, unless otherwise specified, in no

case shall the nominal diameter be less than .001 inch and absolute diameter is less than .0009 inch (see

3.5.2 Protective housing or enclosure. The resistance element of completed resistors shall be

protected by a housing, or an enclosure, or both, which shall be free from holes, fissures, chips, or other

faults, and shall be such as to minimize the establishment of leakage paths between the terminals

resulting from collection of moisture film on the exterior surface of the housing or enclosure. If the

housing is made from a metal or metal alloy, it shall be properly protected against corrosion and all

fasteners shall be plated. Unplated copper-alloy metals shall not be used in contact with aluminum.

3.5.3 Terminals. Terminals shall be as specified in table II. All terminals shall be fastened securely.

Pressure type connections between end terminals and the resistance element shall not be used.

Terminals shall be coated to meet the solderability requirements of 3.18 (see 6.12).