Connector Design Checklist

A good connector design requires attention to the separable interface front end, the permanent interface back end, and the contact retention area. Bill Garver of APEX Electrical Interconnection Consultants provides a checklist of these design considerations.

 

 

There are fundamentally three specific elements of an electrical contact that are used within a multiple-position connector. A multiple-position connector, for this discussion, is defined as one which contains a molded plastic housing and a series of contacts that are attached to wires or printed circuit boards and installed into the molded housing. The contacts may be installed either before (insulation displacement) or after (crimp-snap) the wires have been attached. The contact elements are:

• Separable interface front end
• Permanent interface back end
• Contact retention area

Here we detail several of the important design considerations for each of the three elements.

The following are a few of the critical design attributes, which should be considered for the separable interface front end:

• Contact base material
• Contact plating
• Allowable mating force
• Contact lubricant
• Surface finish (roughness)
• Geometry of contact surfaces
• Closed entry of contacts
• Lead-in condition
• Insertion depth
• Anti-overstress design
• Contact compliance
• Mechanical support of contact beams
• Contact mating sequence
• Contact bifurcation

Let’s look at each of the above design attributes and the selection criteria.

Contact Base Metals

• Steel – used in high-temperature applications
• Copper – used in high-current applications
• Brass – used in low-stress applications
• Phosphor bronze – used in higher-stress applications (most popular)
• Beryllium copper – excellent spring properties, more expensive, environmental concerns

Contact Plating

In making a plating selection, it is important to consider the number of mating cycles, the allowable mating forces, and the application environment in terms of mechanical (vibration and fretting corrosion), thermal (absolute and variable conditions), and chemical (humidity, chlorine, sulfur, oxygen, etc.) factors.

Some of the most popular plating materials and their unique features are discussed below:

Tin

• Minimum thickness = .000120 inches
• Subject to fretting corrosion
• Subject to intermetallic growth at elevated temperatures
• Limited mating cycles (10)
• Subject to whisker growth

Tin/Lead

• Similar to tin except lead prevents whisker growth
• Environmental concerns may limit use

Gold

• Most noble finish
• Excellent electrical and thermal properties
• Thickness depends on durability requirements
• Requires underplate barrier (nickel)
• Pores may allow surface “creeping corrosion”
• High cost

Palladium-nickel

• Good for high-durability requirements
• Good electrical and thermal properties
• Thickness = .000002 – .000005 inches
• Requires underplate barrier (nickel)
• Gold flash overplate reduces corrosion
• May allow surface polymer formation
• High cost

Nickel

• Used in high-temperature applications
• Requires high normal force
• Minimum thickness = .000050 inches
• Susceptible to fretting corrosion
• Subject to galling
• High life cycle with lubrication

Silver

• Excellent electrical and thermal properties
• Used in high-current applications
• Resists contact welding (hot mating)
• Thickness = .000100 – .000300 inches
• May tarnish (reacts with sulfur and chloride)
• Susceptible to surface migration

A number of other issues need to be considered early in the design process. They include:

Contact Lubricants

• Reduce friction
• Increase durability
• Provide corrosion protection
• Act as pore blockers
• May allow thinner plating (cost savings)
• May trap debris
• May be removed during cleaning

Surface finish (roughness)

• Maximum = .000012 inches RMS
• Rolled versus sheared edge is preferred
• Stamping burr should be away from mating surface

Geometry of contact surfaces

• Convex form to each other (crossed-rods shape or convex mating to flat surface)
• Single wear track preferred

Closed entry of contacts

• Best designs prevent contacts from “scooping” and “stubbing”
• Important for customer and blind-mating applications

Lead-in condition

• Should be smooth
• No burrs
• Ensure plastic does not wipe onto contact interface
• Glass-filled plastic may scratch interface

Anti-overstress design

• All designs should include this feature
• Important for customer mating and unmating

Contact compliance (ability to absorb tolerances)

• Should be tested with maximum interference followed by minimum interference
• PCB-mounted connectors should be soldered to a PCB prior to testing

 Mechanical support of contact beams

• Metal is better than plastic
• Must ensure that stress relaxation will not reduce contact normal force below minimum levels

 Contact mating sequence

• Allows contacts to mate and break in a sequence
• Generally “ground-signal-power-sense” circuit mating sequence
• May be accomplished by pin length, PCB trace, etc.
• Must function with all tolerance conditions

Contact bifurcation

• Used to obtain redundant electrical paths
• Good in “dirty” environments
• Reduces individual contact normal force
• One high force contact is preferred to two questionable contacts with lower normal force

The following attributes pertain to the permanent interface backend. Various wire and printed circuit attachment techniques are reviewed along with the associated plating considerations.

Mechanical/Stored Energy

Crimp

• No plating is acceptable
• Gold is good
• Tin is best – lubricates tooling and fills voids

Insulation displacement (IDC)

• Tin is best – wire insertion lubrication and fills void
• Gold – not recommended with tin wire

Compliant pin

• Tin or tin-lead over nickel
• Never gold to tin

Wire wrap

• Tin is best

Metallurgical

Soldered (wave)

• Tin-lead or tin is best
• Nickel or copper underplating extends shelf life
• Palladium is acceptable
• Gold is OK – may poison solder bath over time

Soldered (SMT)

• Tin-lead or matte tin (.000060 inches thick)
• No brighteners
• Nickel underplate (.000050 min inches thick)
• Never gold – forms brittle intermetallics in solder joint

Welded

Resistance

• No plating is acceptable

Laser

• No plating is acceptable

The final element is the retention contact area. Techniques to achieve contact retention include:

• Contact spring latches – latches stamped from the contact body
• Housing spring latches – molded as an integral part of the housing; allows “clean” or smooth contacts to prevent tangling
• Physical entrapment – contacts trapped in a molded cavity in the housing
• Contact barbs (press-fit)
• Insert-molded

A few other considerations regarding retention are:

• Minimum contact retention to housing should be specified
• Push-out forces for pins
• Pull-out forces for crimp/snap cable contacts
• Security to the printed circuit board for compliant pin applications
• Secondary locks are desirable where possible

This is just a broad overview of some of the important design considerations and attributes of contacts that are used within a multiple-position housing. This article may be used as a general guide to ensure that most pertinent design options are considered.

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Bill Garver of Apex Electrical Interconnection Consultants has 47 years experience in the connector industry, primarily in the management and direction of new product development and operational division management. He held the titles of division manager and director of development engineering at AMP. Garver developed new products throughout the full product life cycle, concept through introduction, for numerous industries including consumer, commercial, computer, industrial, communications, and medical.