How to Achieve Perfect PCB Soldering: Physics, Processes, and Best Practices

Soldering is often misunderstood as simply “gluing” two pieces of metal together on the surface. In reality, PCB soldering is a complex physico-chemical process governed by thermodynamics, surface energy, and metallurgy.

For PCB design engineers and hardware startups, mastering the science behind a reliable solder joint is often the decisive factor between a prototype that fails in the field and a product that scales smoothly into mass production.

During prototyping, engineers frequently rely on manual PCB soldering for quick iteration and debugging. However, when it comes to high-reliability electronics, consistent results can only be achieved through controlled industrial processes such as Surface Mount Technology (SMT). JLCPCB bridges this gap by offering industrial-grade SMT assembly services that eliminate human variability while maintaining production-level quality.

That said, a solid understanding of PCB soldering fundamentals remains essential for every engineer. Knowing how solder joints form—and why they fail—enables better design-for-manufacturing (DFM), improves yield, and allows engineers to troubleshoot assembly issues efficiently throughout the entire product lifecycle.

The Physics Behind Perfect PCB Soldering: Wetting and Intermetallics

To achieve a reliable solder joint, two fundamental physical events must occur: Wetting and the formation of an Intermetallic Compound (IMC).

1. The Wetting Angle  (𝜽)

Wetting is the term for how liquid solder distributes on a solid surface (the PCB pad and the component lead). The competition of the surface tension forces governs the behavior.

● Good Wetting (𝜽 < 90°): The solder spreads completely and creates a smooth concave fillet (meniscus). This means high surface energy on the pad (clean copper) and low surface tension in the solder (helped by flux) have taken place.

● Poor Wetting (𝜽 > 90°): The solder forms a bead as water does on a waxed surface. This convex shape, often due to oxidation or too little heat, indicates a weak joint.

2. Intermetallic Compound (IMC) Formation

Soldering is a series of chemical reactions. When molten tin (Sn) comes into contact with copper (Cu), it dissolves a little copper to create a new alloy layer, usually Cu6Sn5.

● The "Goldilocks" Zone: A joint will only form if the IMC layer (1-3µm thick) is maintained. The downside is that with prolonged heating, the IMC layer grows too thick and brittle, and thus becomes a fracture point under vibrations.

● The Role of Flux: Molten solder cannot wet oxidized copper. Flux acts as a reducing agent, stripping oxides at high temperatures to expose the active metal for IMC formation.

Cross-section of a PCB solder joint

Industrial PCB Soldering Methods: Reflow and Wave Soldering

Although manual soldering is still used for prototyping, JLCPCB's SMT assembly service relies on fully automated processes to ensure consistent solder joint quality across high-volume production.

Reflow Soldering – The SMT Standard for PCB Assembly

Reflow soldering involves printing solder paste onto pads, placing components, and passing the assembly through a convection oven with a precisely controlled thermal profile.

The Thermal Profile: Four Critical Zones

1. Preheat (Ambient to ~150°C): This step evaporates the solvents and, at the same time, slowly heats the PCB to avoid thermal shock. Ramping too fast (>3°C/sec) causes component cracking.

2. Soak (150°C to ~180°C): Achieves thermal equilibrium across the board. Flux activates here, scrubbing oxides.

3. Reflow (Time Above Liquidus - TAL): The solder reaches its maximum melting temperature(~245°C for SAC305) during this stage and melts completely.

4. Cooling: Quick cooling leads to a fine-grain structure, which is the cause of excellent mechanical strength.

Standard SMT reflow soldering thermal profile for SAC305 lead-free solder

Wave Soldering – Through-Hole PCB Soldering

Used for Through-Hole Technology (THT). The PCB passes over a fountain of molten solder.

Limitation: The SMT parts located at the bottom have to be glued. Nowadays, designers frequently choose Selective Soldering in order to prevent thermal shock to the SMD components.

Comparison: Reflow Soldering vs. Wave Soldering

Manual PCB Soldering Techniques for Prototyping and Rework

Every engineer needs manual soldering skills for R&D. The underlying physics of soldering remain unchanged; only the heat delivery mechanism differs when using a soldering iron.

1. Thermal Dynamics of the Soldering Iron

Thermal Recovery: A 350°C iron with a tiny tip will drop to 200°C instantly when touching a ground plane. You need an iron with high thermal capacity/recovery, not just high temperature.

Guidelines:

● Sn63Pb37 (Leaded): ~315°C - 340°C.

● SAC305 (Lead-Free): 350°C - 375°C. Never exceed 400°C.

2. The Heat Bridge Technique Explained

Improving Thermal Coupling Using Molten Solder (“Heat Bridge” Technique)

The Problem: The Dry iron tip has very little thermal contact with the lead/pad.

The Solution: Prior to touching the joint, the tip gets a tiny blob of fresh solder. The liquid solders fill in the air gaps and form a Heat Bridge that transfers the heat instantly.

The soldering heat bridge technique, showing how applying a small amount of molten solder to the iron tip improves thermal transfer to the PCB pad.

3. The 3-Step Manual PCB Soldering Process

Heat: Iron touches pad + lead (1 sec).

Feed: Solder wire enters the joint (not the iron).

Withdraw: Remove wire, then iron.

3-step manual soldering process

Soldering Iron Tip Selection

Soldering Materials That Enable Perfect PCB Soldering

The materials you select indicate the process window.

● SAC305 (Lead-Free): The most common in the industry (Sn-Ag-Cu). Melts at around 217°C. The appearance of the joints is duller and grainier than in the case of leaded solder.

● Sn63Pb37 (Leaded): Eutectic (melts at 183°C). Mainly used for prototypes because of easy wetting, but banned in most commercial products (RoHS) owing to toxicity.

Surface Finishes:

● HASL: Low cost, sturdy, but rough surface. Not good for fine-pitch.

● ENIG: Nickel/Gold plating. Totally smooth, that is, it is a must for BGAs and QFNs.

● JLCPCB Tip: Go for ENIG in any design with <0.5mm pitch components to have consistent planarity.

Learn More: Comparing HASL and ENIG Surface Finishes for PCBs

Design for Assembly (DFA): Designing PCBs for Perfect Soldering

Perfect soldering starts in CAD.

● Thermal Relief: Never connect a pad to a ground plane directly; the plane will act as a heat sink and keep the solder from melting. Use thermal spokes to isolate the pad and still have an electrical connection.

● Stencil Aperture: JLCPCB makes use of the finest LPKF laser-cut stencils. The aperture size for the fine-pitch parts is usually reduced to 85-90% of the pad size to avoid bridging.

● Pad Definition (NSMD): With BGAs, Non-Solder Mask Defined (NSMD) pads (mask opening > pad size) are recommended. This allows solder to wrap around the copper pad edge, thereby increasing the grip.

Comparison of PCB layout, showing a component pad mounted to a ground plane, and the use of thermal relief spokes to avoid heat sinking.

Common PCB Soldering Defects and How to Prevent Them

1. Cold Joint: Matte, uneven. Resulting from a lack of heat or motion at the time of cooling.

2. Tombstoning: Component stands vertically. Caused by unequal wetting forces (one pad heats faster than the other).

3. Bridging: A Liquid connection between two pins. Resulting from too much paste or not enough solder mask dams.

4. Voiding: There are gas pockets inside the joint. The reason for this is trapped flux outgassing (insufficient soak time).

Comparing a perfect concave solder fillet against common PCB soldering defects, including cold solder joints and tombstoning.

Advanced PCB Soldering Techniques and Reliability Considerations

Selective Soldering & Rework:

For hybrid circuit boards, Selective Soldering applies a programmed mini-wave nozzle to attach particular THT pins without directly affecting the nearby SMT parts' heat sensitivity. When repairing BGAs, restrict the board to 3 reflow cycles at most to avoid both FR4 material degradation and gradual growth of the brittle IMC layer.

RoHS and Flux Residue:

● RoHS Compliance: The EU directives impose the use of lead-free (SAC305) for the vast majority of electronic devices, thus making RoHS compliance a necessary evil.

● Flux Cleaning: "No-clean" flux leaves inert residue, but high-reliability (Class 3) aerospace/medical boards often require Water-Soluble flux followed by automated washing to remove all corrosive residues.

Quality Control and Inspection for High-Reliability PCB Soldering

Visual inspection isn't enough for modern high-density boards.

● Automated Optical Inspection (AOI): Cameras scan for missing parts, polarity errors, and bridges. JLCPCB uses AOI on all SMT orders.

● X-Ray Inspection: The only way to see under BGAs and QFNs. X-rays reveal voiding percentage (IPC limit <25%) and "head-in-pillow" defects where the ball didn't merge with the paste.

X-ray inspection of a BGA component showing the distinction between good solder balls and voiding / bridging

Conclusion

The high-quality PCB soldering is the combination of physics and process control. Lab-only "Heat Bridge" technique is a must, while in mass production, success is very much dependent on the accurate thermal profiling and automated inspection of the industrial SMT lines.

JLCPCB uses advanced AOI, X-ray testing, and processes to provide reliability. Whether prototyping or scaling up to production, knowing these principles will make your hardware work without any problems.

Ready to scale? Upload your Gerber files to JLCPCB for high-reliability SMT assembly.

FAQs about PCB Soldering

Q1: What is the optimal temperature setting for the soldering iron in the case of lead-free PCB?

For such alloys as SAC305, the tip temperature should range from 350°C to 375°C. Although lead-free solder has a melting point of about 217°C, the additional heat is mainly to cover the heat lost to the board and also to ensure a good thermal recovery when soldering pads connected to ground planes. These are the tip temperatures—the joint reached is 30-50°C lower. If the tip temperature goes over 400°C, the pads will be damaged, and the flux will be burnt instantly.

Q2: Why is flux necessary for soldering?

Flux plays three major roles: it cleans the metal surfaces chemically by removing the oxides (reduction reaction), and does not let them re-oxidize during the high temperature phase, and lowers the surface tension so that molten solder can flow and "wet" the pad nicely. In the case of no flux, solder would form beads on oxidized copper instead of creating the intermetallic compound required for good bonding.

Q3: What is the difference between reflow soldering and wave soldering?

Reflow soldering uses solder paste and a convection oven to attach Surface Mount (SMT) components through a controlled four-zone thermal profile; heat comes from hot air circulating the board. Wave soldering passes the PCB over a molten bath of liquid solder and is primarily used for Through-Hole (THT) components. Reflow offers superior control for fine-pitch parts, while wave soldering efficiently handles connectors and power components.

Q4: Can I combine manual soldering with JLCPCB's SMT service?

Absolutely, this is a normal and budget-friendly workflow. Get JLCPCB to do the assembly of all complex SMT parts (BGAs, fine-pitch ICs, 0402 passives) through automated processes, and then manually solder through-hole connectors or non-standard components in your lab. Such a setup helps you get rid of tooling costs for simple through-hole components while still ensuring precision.

Q5: How do I prevent tombstoning on small SMT components?

Tombstoning is produced when one pad during reflow becomes hotter than the other, thus generating different wetting forces. You can avoid it by making thermal connections to both pads symmetrical, balancing trace widths, adjusting your soak zone for thermal equilibrium across the board, and using solder paste deposits of the right size. The wider plastic range of SAC305 compared to some alloys also plays a role in minimizing the occurrence of tombstoning by equalizing the wetting force.