MIL-PRF-49465C
6.3 Qualification. With respect to products requiring qualification, awards will be made only for products which are,
at the time of award of contract, qualified for inclusion in the Qualified Products List whether or not such products
have actually been so listed by that date. The attention of the contractors is called to these requirements, and
manufacturers are urged to arrange to have the products that they propose to offer to the Federal Government tested
for qualification in order that they may be eligible to be awarded contracts or orders for the products covered by this
specification. Information pertaining to qualification of products may be obtained from Defense Supply Center,
Columbus, ATTN: DSCC-VQP, P.O. Box 3990, Columbus, Ohio 43218-3990
6.4 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).
6.5 Derating. The intention of this specification is to cover resistors capable of full load operation at any ambient
temperature up to and including +25OC. However, if it is desired to operate these resistors at ambient temperatures
greater than +25OC, the resistors are derated in accordance with figure 1. For efficient and long life operation,
resistors are derated by more than 50 percent.
6.6 Mounting. Under conditions of severe shock or vibration, or a combination of both, resistors of all sizes
described in this specification are mounted in such a fashion that the body of the resistor is restrained from
movement with respect to the mounting base. It should be noted that if clamps are used, certain electrical
characteristics of the resistor may be altered. The heat dissipating qualities of the resistor will be enhanced or
retarded depending upon whether the clamping material is a good or poor heat conductor. Under less severe
vibration conditions, axial lead styles may be supported by their leads only. The lead lengths should be kept as short
as possible, .250 inch or less preferred, but not longer than .625 inch. The longer the lead, the more likely that a
mechanical failure will occur.
6.7 Power dissipation. When higher ambient temperatures exist or when resistors are mounted in enclosures
which limit ventilation, the wattage dissipation of any resistor should be reduced so that the maximum hot spot
temperature permissible for the resistor is never exceeded under the most severe combination of temperature
conditions.
6.8 Spacing. When resistors are mounted in rows or banks, they should be so spaced that, taking into
consideration the restricted ventilation and heat dissipation by the nearby resistors, none of the resistors in the bank
or row exceeds its maximum permissible hot spot temperature. An appropriate combination of resistor spacing and
resistor power rating must be chosen if this is to be assured.
6.9 Secondary insulation. Where high voltages are present between resistor circuits and grounded surfaces on
which resistors are mounted, secondary insulation capable of withstanding the voltage conditions should be provided
between resistors and mountings or between mountings and ground.
6.10 Choice of styles. The styles of resistors to be used in equipment should be chosen that, when mounted in the
equipment, they will not operate at a temperature in excess of their rating. This should be applicable under the worst
possible specified conditions, with the equipment operating as follows:
a.
In the maximum specified ambient temperature.
b.
Under conditions producing maximum temperature rise in each resistor.
c.
For a sufficient length of time to produce maximum temperature rise, or for the maximum specified time.
d.
With all enclosures in place.
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