Crimp vs Solder vs PC Tail:
How to Choose Contact Terminations for MIL-DTL-38999

One Connector Shell, Five Ways to Wire It

You've selected the right MIL-DTL-38999 connector — Series III, shell size F, 35 contacts, electroless nickel finish. Then the RFQ asks: What contact type? Pins or sockets is the easy part. But crimp, solder cup, PC tail, eyelet, or hermetic feedthrough? That choice determines your assembly process, tooling investment, field repairability, vibration resistance, and — in the case of hermetic connectors — whether your pressure vessel leaks at 30,000 feet.

Too many engineers default to "crimp" without understanding the trade-offs, or select "solder" because it feels familiar from PCB prototyping — unaware that IPC/WHMA-A-620 Class 3 explicitly requires crimped terminations for aerospace wiring and that solder joints under vibration are a known field-failure mechanism. This guide breaks down all five MIL-DTL-38999 contact termination types with a practical decision framework you can use on your next design review.

The Five Contact Termination Types at a Glance

Crimp Contacts: The Aerospace Standard

Crimp contacts — specifically AS39029 rear-release, rear-insertable contacts — are the dominant termination method across all MIL-DTL-38999 applications. They're specified by NASA, the DLA, and every major OEM because a properly executed crimp creates a cold-welded, gas-tight joint between the wire conductor and the contact barrel that outperforms solder in both electrical and mechanical reliability.

Why Crimp Dominates

• IPC/WHMA-A-620 Class 3 mandated: Aerospace and defense wiring standards explicitly require crimped terminations — solder is not an acceptable alternative for flight-critical harnesses
• Vibration-proof: A gas-tight crimp has no solder fillet to fatigue and crack. D38999 Series III with crimp contacts survives 40g random vibration at 10–2000 Hz
• Field-repairable: Damaged contacts can be extracted from the rear using a plastic extraction tool, replaced, and re-inserted — no de-soldering, no connector replacement
• Consistent quality: Ratcheting crimp tools with Mil-Spec positioners (e.g., DMC AFM8, Daniels M22520/1-01) produce identical results every cycle — operator skill is less of a variable than with hand soldering
• Contact density: Crimp contacts support up to 128 positions in shell size J — the highest density available in 38999

The Downside: Tooling Investment

The primary barrier to crimp is tooling. A single MIL-SPEC crimp tool like the DMC AFM8 with a turret head runs $800–$1,200. Full-size frame tools (M22520/1-01) with positioners for multiple contact sizes can push past $2,500. Each contact size (#22D, #20, #16, #12, #8) requires its own positioner or die set. For a production line building harnesses with mixed contact sizes, the upfront tooling budget can reach $4,000–$6,000 before the first connector ships.

However, for any volume above ~100 connectors, this cost amortizes quickly. At $0.30–$0.80 per crimp contact (vs. $0.15–$0.40 for solder-type), the per-unit delta is negligible against the labor savings and reliability improvement.

Solder Cup Contacts: When Cost or Geometry Wins

Solder cup contacts are the original termination method — a small cup at the rear of the contact into which a stripped wire is inserted and soldered. They remain relevant in three specific scenarios:

Critical Rule: Never Crimp AND Solder

A common misconception in field repairs is that adding solder to a crimped joint improves reliability. The opposite is true. Solder wicks up the wire strands inside the crimp barrel, creating a rigid point where the wire transitions from flexible conductor to stiff, solder-filled conductor. Under vibration, this transition point becomes a stress concentrator — the wire fractures exactly where the solder stops wicking. NASA-STD-8739.4A explicitly prohibits soldering a crimped termination. One or the other. Never both.

PC Tail Contacts: Bridge to the PCB

PC tail contacts feature an extended termination pin protruding from the rear of the connector body, designed to pass through plated through-holes on a printed circuit board. They're the standard choice for box-mount and panel-mount receptacles where the connector is permanently affixed to an avionics chassis, a radar processing unit, or a satellite bus electronics enclosure.

PC Tail Selection Checklist

One design caution specific to PC tails: routing density. A shell size G receptacle with 55 PC tails poured through a PCB creates a dense pin field that can complicate trace routing on inner layers. Consider using a jam-nut receptacle with standoffs to create a dedicated connector zone on the board, and avoid placing tall SMT components within 0.300" of the connector footprint to leave room for the insertion/extraction tool path.

Eyelet Contacts: For Direct Wire Attachment

Eyelet contacts (D38999 code X/Y for hermetic) are less common but indispensable in specific applications. They feature a small hollow cylinder at the rear — similar to a PC tail with a hole through the center — designed for direct wire insertion and soldering. Their primary use case is inside hermetically sealed enclosures where the connector's rear face is exposed to the internal environment (vacuum, inert gas, or controlled atmosphere). Wires are hand-soldered through the eyelet using MIL-SPEC solder and inspected under magnification per NASA-STD-8739.4A.

Decision Matrix: Which Termination Should You Use?

Cost Comparison: Crimp vs Solder vs PC Tail

Airoadcon Contact Options: What We Ship

Airoadcon's J599 Series III (MIL-DTL-38999 equivalent) connectors are available with all five contact termination types across our full product range:

• Crimp (AS39029): Standard on all J599 environmental connectors — rear-release, gold-plated copper alloy, sizes #22D through #8. Compatible with all MIL-SPEC crimp tools (DMC AFM8, Daniels M22520 series, Astro, Sargent).
• Solder cup: Available on special order for prototype builds and hermetic J599H connectors. Pre-tinned cups for easy wetting.
• PC tail: Standard offering for box-mount and jam-nut J599 receptacles. Custom tail lengths available (0.125" to 0.750").
• Eyelet: Standard on J599H hermetic connectors — types X (pin) and Y (socket) with 0.040" eyelet diameters.
• 1500-cycle high-durability: Available for test interfaces and connector savers where mating cycles exceed standard 500-cycle rating.

Every contact ships with full lot traceability and CoC documentation — same quality system that produces our DLA QPL-listed product lines.