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"Mil-spec" and NASA-spec

Started by teemuk, October 18, 2008, 10:19:12 AM

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teemuk

The following is an excerption of something I've been working on for adding into the 2nd edition of the amp book. I though it might interest some of you.


When it comes to wiring and construction the issue of using "mil-spec" or "NASA-spec" techniques often comes up. The aforementioned are actually based on to tedious standards issued in various books of standards that typically have anything from 600 to 1000 pages discussing topics such as component materials and construction methods in extreme detail. If one goes through such book, or even just a brief summary of one, it often comes evident that in most cases many amplifiers claiming to employ "mil-spec" construction often actually do not. For example, the popular style of "hanging" components in between turrets, using 90-degree angles (Hiwatt anyone?) in wiring and boosting serviceability by securing components to terminals only by means of solder is not being "mil-spec". (However, that does not mean that these amplifiers weren't built much better than an average one).

The following is a brief summary of topics from NASA's construction standard that relates to generic construction issues one would likely deal with when building guitar amplifiers. If reference to a certain standard is not mentioned, the method is optional but considered as "Best Workmanship Practice" which preferably should be used. One should also bear in mind that this small list of things is just the tip of an iceberg.

Crimped terminations:
-   Crimping of component leads, solid wire or leaded/tinned stranded wire is prohibited. (NASA-STD-8739.4)
-   The crimp termination must have some clearance to wire's insulation. However, this exposed part will be insulated with heatshrink tubing or by another similar method. (NASA-STD-8739.4)
-   Discoloration or charring in wiring, or the use of charred/damaged heatshrink is prohibited. (NASA-STD-8739.4)
-   The crimp must be intact and it is unacceptable to modify one "to fit". (NASA-STD-8739.4)

Solderless wire-wraps:
-   Made by helically wrapping solid, uninsulated wire around a specially designed termination post. The wrap post usually has a rectangular shape.
-   Use of stranded wire is prohibited. Also is prohibited the use of silver underplating. (MIL-STD-1130B)
-   Half to one and a half turns of insulated wire must be in contact with a minimum of three corners of the wrap post. Absence of insulated turns is prohibited. (MIL-STD-1130B)
-   In single termination, "the termination has a specified amount of insulated and uninsulated turns of wire, and is clean and free of foreign material." (MIL-STD-1130B)
-   In multiple terminations, the terminations preferably have a proper amount of spacing. The terminations can overlap but in this case the insulated conductor overwrap does not exceed one turn and is as tight as possible. (MIL-STD-1130B)
-   Insufficient insulation wrap, improper wrapping position, insufficient number of turns in a wrap, "end tails", overwraps, spiral wraps, "open" wraps, improper routing of wires from one post to another, as well as using damaged wrap posts is unacceptable. (MIL-STD-1130B)

Cables and harness:
-   "Cables insulated with materials other than Kapton shall not be bent less than six outer diameters." "Flat and ribbon cables shall not be creased, folded or bent less than three insulated wire diameters (short-term)." (NASA-STD-8739.4). Therefore the 90-degree bend you often see used is unacceptable, at least by NASA's standards.
-   Ribbon cables should not be incorporated to discrete wiring harnesses. Ribbon cables should be routed along flat surfaces. They should not be routed near heat, electric interference or vibration sources. Naturally blocking airflow is prohibited.
-   Splices (meaning joining of two or more electrical conductor wires) must have proper strain relieves and must be located in areas that are not subjected to flexure. The splice connection must follow the standard, being either a crimped splice connector, lash splice or a Western Union/Lineman splice for solid conductors. (NASA-STD-8739.3)

Miscellaneous:
-   Solder joints are clean of dirt and foreign substances, shiny, smooth and concave. Plated-through holes are filled completely with tin. Leads must protrude properly through the holes but not too much. Wire insulation is not allowed to extend to solder joint. Clinched (bent) lead ends shall not be clinched towards an electric conductor or a proper clearance to such must exist.
-   Preferably solder should not extend to stress relief bends but it is acceptable if the topside bend radius is discernable. Solder joint must not extend to component body and clearance of at least one lead diameter is required. (NASA-STD-8739.3)
-   Gold-plated surfaces that become part of a solder joint must be tinned. Gold "intermetallic" (golden stripes in solder) will make the solder joint brittle.
-   Part bodies must not be in contact with soldered terminations.
-   Glass encased parts (e.g. those small diodes) must have a sleeve or must be potted with transparent epoxy. (NASA-STD-8739.3)
-   Heat-producing parts must have sufficient clearing to circuit board.
-   Components that weight more than 7 grams must be mechanically secured on their place. If hot glue (or similar substance) is used for this purpose the mounting must be done with at least four evenly spaced bonds.
-   Eyelets should not be used for interfacial terminations.
-   In lead bending, "the minimum distance from the part body/seal to the start of the bend shall be 2 lead diameters for round leads and 0.5 mm for ribbon leads. The bend radius shall not be less than one lead diameter or ribbon thickness". (NASA-STD-8739.3)
-   The component should be centred between its mounting holes.
-   Splices are not allowed in repairing broken or damaged conductors or part leads.
-   Broken components or tracks, flux splatters and residue, as well as excess solder or discoloured laminates are unacceptable.
-   In case of axially mounted components, use stress relieve whenever possible. Component body should preferably rest in full contact with the mounting surface. Slight gap is acceptable but in case of large caps the component must be mechanically secured on its place. Leads of axially mounted components are allowed to cross exposed conductors, but in this case the leads must be sleeved.

Leads of axial components should be equipped with strain relief "loop" that acts like a spring. The body of the component is firmly mounted on its place (often with glue or cable straps). When axial components are mounted in radial fashion the proper support depends on the board design: Components used in boards that have no plated-through holes rest on their body and are often glued on place as well. The longer leg has two 90-degree bends that make it stiffer. At solder pads, the leads are angled to establish a mechanically sturdier construction. In boards that have plated-through holes the construction is different: The solder joint is mechanically rigid and also needs some clearance at the component side. The longer leg is hence angled to make it spring-like. The component body is left "floating" in air. This method of construction is not very sturdy! Axial components can also be mounted in radial fashion. In this case their leads must have proper strain relief and their bodies should be mechanically secured to the mounting surface.

Discrete wires:
-   Discrete wires are subject to same rules as component leads when it comes to bending, stress relief and soldering requirements.
-   In PC boards, conductor enters the hole perpendicular to board surface. Proper insulation clearances and lead protrusions are used. Excessive insulation gaps (more than two wire diameters) are unacceptable. (NASA-STD-8739.3)
-   No more than one component shall use the same hole.
-   In "lapped termination" (no mounting hole, wire simply soldered to a pad) the termination must run across the longest dimension of the pad. The conductor is not allowed to overhang across the pad. (NASA-STD-8739.3)

Jumper wires also known as "haywires":
-   The jumper wire must be solid, insulated copper conductor with tin/lead plating. Stranded, silver-plated wiring is not allowed. Jumpers less than 25mm in length may be uninsulated if short circuit to adjacent components is impossible.
-   In component side of PC boards, wire route must be the shortest path and does not pass over or under components or over any land or via used as a test point. The jumper wire must have sufficient slack to allow component replacements.
-   In solder side of PC boards, wire route is the shortest path and does not pass over component footprints or lands – except if unavoidable.
-   The wire must be "staked" (securely tied on its place to restrict movement) on specified intervals and in all places where it changes its direction. (NASA-STD-8739.1)
-   Stress relief is used whenever possible. Improper strain relief is unacceptable. (NASA-STD-8739.3)

Solder terminals:
-   Solder is not mechanically sturdy so the wire must first pass through the "eye" (hole) of the terminal and then be secured with hook wrap (minimum of 180-degree wrap), quarter turn wrap (90-degree wrap in contact with terminal face) or a "zig-zag" wrap (two 90-degree wraps in contact with both sides of the terminal face).
-   Soldering the wire to terminal without additional and standardized securing is unacceptable. (NASA-STD-8739.3)
-   Wire runs from one terminal to another must have a strain relief and proper, standardized wrap contact at each terminal. Wire ends must be wrapped 90 or 180 degrees. (NASA-STD-8739.3). Turret board terminals are an exception to the rule and explained later.

Turret board construction:
-   Wires are and component leads are wrapped a minimum of 180 degrees around the turret. Multiple wires or leads may connect the same turret but they must run in parallel, wraps may not overlap and at least one of the wires has to have a contact to terminal base. Wires are soldered on place and they are discernable under the solder joint. The solder joint is naturally shiny and smooth. (NASA-STD-8739.3)
-   Conductor sizes AWG 26 and smaller must be wrapped a minimum of 180 degrees (1/2 turn), but less than one full turn (360°) around the post. Conductor sizes larger than AWG 26 must be wrapped a minimum of 180 degrees, to a maximum of 270 degrees (3/4 turn) around the post. (NASA-STD-8739.3)
-   The wrapping can be either into clockwise or counter-clockwise direction. Important is that the wrap would be tightened against the terminal would the wire become pulled. (NASA-STD-8739.3)
-   In continuous wire runs from one terminal to another, "the wire shall wrap (360 degrees) around each terminal, contact each terminal base, exhibit stress relief, and be terminated to the first and last terminal with a 180° to 270° wrap (depending on wire gauge)." (NASA-STD-8739.3)
-   Mounting components on top of turrets is unacceptable! (NASA-STD-8739.3)
-   Parallel components may be stacked on top of each other but the largest component must be at the bottom and rest against the mounting surface. In this case, all components must be stress-relieved and secured mechanically on place.

Jack1962

Thanx Teemuk, great info.  :tu: