In electronics manufacturing, soldering is not just the physical link between components and the printed circuit board (PCB). It also governs electrical performance, mechanical strength, and long-term reliability. As aerospace, defense, and high-end communications impose increasingly stringent reliability requirements, simply "getting it soldered" is no longer sufficient. Engineers must understand the microstructure and load path inside a solder joint to design and control processes that consistently meet mission-critical expectations.
This article reviews the evolution of solder joint structures from single-sided boards without plated-through holes to double-sided plated-through-hole (PTH) PCBs. It analyzes how lead protrusion height, wetting height, and the intermetallic compound (IMC) layer inside the hole affect reliability. Drawing on standards such as IPC-A-610C/H and MIL-S-45743E, it offers a practical, standards-based framework for process optimization and quality control in wave soldering.
From Single-Sided to Double-Sided PTH: How Joint Structure Evolved
Reliability starts with the substrate and interconnect architecture. Early products frequently used single-sided PCBs with non-plated holes, while modern high-reliability systems predominantly adopt double-sided boards with plated-through holes. These two structures differ fundamentally in how the solder joint forms and how it performs over time.
Single-Sided PCBs without Plated Holes: Structure and Limitations
On a single-sided PCB without plated-through holes, the solder joint exists only on the surface. There is no solder-filled hole structure. Reliability depends heavily on the adhesion between pad copper and the substrate, the solder wetting height (h), and the thickness/quality of the intermetallic layer at the copper–solder interface. Lacking a through-thickness metallurgical connection, this structure exhibits inherent limitations in both mechanical strength and electrical performance.
To improve reliability on such boards, the following measures are commonly applied:
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Use structural reinforcement in the mounting: Optimize the mechanical mounting so the component's self-weight (Fg) or external forces (F) are not borne by the pad copper alone, preventing pad lifting or copper foil peel-off.
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Control lead protrusion height and wetting height: Experiments by Japanese researcher Eiichi Tsunashima showed that for single-sided boards, a lead protrusion height H of 3.18 mm (1/8 inch) yields the highest joint strength. Increasing solder wetting height h enlarges the contact area, improving tensile strength. Boeing and IPC-A-610 standards both constrain H and h; for example, IPC specifies that on a single-sided board the protrusion height should be at least 0.5 mm.
Double-Sided PTH PCBs: Structural Advantages
With the maturation of plating technology, double-sided PTH PCBs have become mainstream due to superior mechanical strength, electrical continuity, and heat conduction. Their solder joint comprises two key regions: the portion inside the hole and the external, exposed portion on the solder side.
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Inside the hole: Ideally, the hole is fully filled with solder. A uniform, dense copper–tin intermetallic compound (IMC) forms between the plated barrel and the lead. This IMC provides a high-strength metallurgical bond and excellent electrical and thermal conductivity, and is central to long-term reliability.
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External exposed portion: This is the portion of the joint protruding from the solder side. While its effect on reliability is less than the inside-hole region, modern processes focus on how it influences wave solder hydrodynamics, distributed parameters in high-frequency circuits, and conformal coating coverage and uniformity.
How External Joint Geometry on PTH Boards Affects Operation
Although the inside-hole metallurgical connection dominates reliability, the exposed portion—especially the lead protrusion height H—still strongly influences wave solder process stability, circuit behavior, and environmental robustness.
Improving Wave Solder Process Stability: Suppressing Turbulence and Defects
Lead protrusion height directly affects the flow of molten solder at the wave crest. Excessive H disrupts the boundary layer, promotes turbulence and vortices, and raises the risk of bridging, solder balls, and blowholes. Experience and production data show that keeping H under 1 mm substantially reduces flow disruption and defect rates—particularly valuable for DIP packages and multiway sockets where leads are tightly spaced.
Enhancing Circuit Stability: Limiting Distributed Capacitance and Stray Fields
In high-frequency circuits, long protruding leads introduce parasitic fields and increase distributed capacitance. These parasitic elements can degrade signal integrity and stability. Consequently, high-frequency products impose tighter limits on H to minimize unwanted parasitics.
Improving Environmental Robustness: Preventing Leakage and Tip Discharge
In high humidity, salt fog, or high-voltage environments, excessive lead length can become a failure initiator:
- Tip discharge: In high-voltage assemblies, sharp, overlong lead tips can concentrate electric fields and trigger corona or tip discharge, risking catastrophic damage.
- Leakage: Historical cases include leakage during damp heat and salt fog testing in a terminal-guidance radar application, traced to overlong leads that prevented adequate conformal coating thickness. Simply shortening the leads enabled the coating to cover effectively without resorting to manual "mound-shaped" solder buildup, which is unacceptable and unreliable.
Optimizing Weight and Space: Meeting Aerospace Constraints
Weight directly affects performance in aerospace systems; for example, adding 1 kg can reduce missile range by kilometers. Weight creep from unnecessarily long leads and excess solder is avoidable. There is precedent where acceptance was initially refused due to weight excess, later rectified by trimming leads. Cultivating "light, thin, short, small" design and process habits helps control mass and volume at the source.
Lead Trimming: Process Rules and Reliability Risk Control
IPC-A-610C permits lead trimming provided it does not introduce mechanical shock. In practice, strict controls are required to avoid new failure modes.
After Trimming, Re-solder the Joint
Post-wave-solder lead cutting is generally discouraged. If trimming is unavoidable, re-soldering the joint is mandatory for two reasons:
- Trimming exposes the base metal of the lead, removing the protective end surface. The cut face is prone to oxidation and corrosion over time, jeopardizing joint integrity.
- Shear forces during cutting can induce microcracks at the lead–solder interface, creating latent fracture initiation sites. A controlled secondary solder operation re-establishes metallurgical continuity and encapsulates the cut end.
The Secondary Solder Is Part of the Process, Not "Rework"
From a process standpoint, the post-trim solder operation should be treated as an integral step in the production flow, subject to the same quality criteria as the primary soldering. This ensures consistent IMC formation, full wetting, and protection of the freshly cut lead surface.
Using a Data Window to Improve Process Control
IPC-A-610H (Class 3) specifies a lead protrusion height H between 0 and 1.5 mm. For better manufacturability and defect prevention, this range can be split into a "lower data window" and an "upper data window," applied differently depending on component type and pad density.
Lower Data Window (0–0.75 mm): For Dense Lead Arrays
Dense-pitch devices such as DIPs and multiway sockets are typically stable in mechanical placement; their primary risk is interference with wave flow. Prefer the 0–0.75 mm range to reduce turbulence and bridging risk, increasing first-pass yield.
Upper Data Window (0.75–1.5 mm): For Isolated Components
For components with few leads (resistors, capacitors, diodes) and isolated pads, wave interference is smaller while positional stability may benefit from extra length. Allowing H closer to 1.5 mm can improve fixturing stability and assembly robustness without compromising solder quality.
Summary
The evolution of PCB solder joint structure reflects a broader shift in manufacturing from "functional connectivity" to "high reliability." Engineers should move beyond the notion that "taller is stronger" and evaluate joint geometry through the lenses of fluid dynamics, electromagnetic behavior, and environmental robustness.
In contemporary wave soldering, the inside-hole metallurgical connection is the foundation of reliability. Optimizing the exposed portion—especially the lead protrusion height—is essential for yield, stability, and environmental performance. By controlling H within a standards-based data window, and by enforcing strict rules for trimming and secondary soldering when needed, manufacturers can achieve reliable, efficient, and lightweight assemblies.
Appendix: Summary of Lead Protrusion Height Requirements in Major Standards
| Standard/Organization | Requirement | Application |
|---|---|---|
| IPC-A-610H (Class 3) | H = 0–1.5 mm; tighter control recommended for high-frequency or high-density designs | High-performance electronics |
| MIL-S-610C | H ≥ 0.5 mm (single-sided) | General electronic assembly |
| MIL-S-45743E | H = 0.76 mm (min), 1.5 mm (max) | Military electronic equipment |
| Boeing internal practice | H ≈ 0.79 mm (1/32 inch) | Aerospace and missile systems |
| Japanese researcher Eiichi Tsunashima | H = 3.18 mm (single-sided, peak mechanical strength) | Traditional single-sided reinforcement |
