Common PCB Assembly Methods and Soldering Techniques Explained

Whether you’re designing your first prototype or scaling up to production, understanding PCB assembly methods and soldering techniques is crucial to achieving reliable, high-performance circuit boards. Modern PCBA primarily relies on Surface Mount Technology (SMT) and Through-Hole Technology (THT)—each offering unique advantages for component density, durability, and manufacturability.

In this guide, we’ll break down the major PCB assembly methods, key soldering techniques such as reflow and wave soldering, and the step-by-step PCB assembly process. You’ll also learn how to identify and fix common defects like tombstoning, voids, and cold joints, helping you build cleaner, more consistent boards.

Main PCB Assembly Methods

PCB assembly is not a single technique but a collection of manufacturing strategies optimized for different component types, reliability requirements, and cost targets. The main PCB assembly approaches include Surface Mount Technology (SMT), Through-Hole Technology (THT), and mixed assembly, each with unique implications for signal integrity, thermal management, and manufacturability.

1. Surface Mount Technology (SMT)

Surface Mount Technology (SMT) dominates modern PCBA because it enables miniaturization and high-density layouts. Instead of long leads, Surface Mount Device (SMD) has flat terminations or solder balls that sit directly on copper pads.

SMT Assembly Process:

1. Stencil Printing - A stainless steel stencil deposits solder paste with precise volume control.
2. Pick-and-Place - High-speed machines place SMDs at ±25 μm accuracy.
3. Reflow Soldering - Thermal profile (preheat, soak, peak ~245 °C, controlled cool-down) forms intermetallic solder joints.
4. Inspection - AOI and X-ray for hidden-joint packages (BGA, QFN).

SMT Assembly Process

Outsourcing SMT assembly can save time, reduce rework, and guarantee precision that’s hard to achieve manually. For engineers and product developers who need professional SMT assembly with automated pick-and-place and reflow capabilities, JLCPCB’s SMT PCB Assembly Service provides full turnkey PCB manufacturing—from stencil printing to component sourcing, assembly, and quality inspection—helping you turn designs into production-ready boards faster and more reliably.

2. Through-Hole Technology (THT)

Through-Hole Technology (THT) refers to the process of inserting component leads into drilled holes that are plated with copper, and then it is soldered either by wave soldering or selective soldering.

THT Assembly Process:

1. Hole Drilling - CNC drilling, plating to create conductive barrels.
2. Component Insertion - Manual or robotic insertion (axial or radial insertion machine).
3. Wave/Selective Soldering - Molten solder bonds leads to barrels.

THT component lead soldered through a PCB hole to create a secure joint

3. Mixed Assembly (SMT + THT)

Most real-world boards will use a combination of SMT and THT:

• SMT for ICs, passives, and RF circuits.
• THT for connectors, power semiconductors, and relays.

Mixed Assembly Process:

1. SMT is placed and reflowed first.
2. THT is inserted afterward, then wave or selective soldering.
3. Conformal coating or underfill protection may also be used to address the ruggedness of the environment.

Challenges of Mixed Assembly (SMT + THT)

• Need to manage two different soldering processes (reflow + wave/selective)
• Thermal management has to address two different solder types.
• Board warpage may introduce alignment issues during the second soldering step.

To simplify this, JLCPCB offers hybrid assembly services, integrating SMT reflow and THT selective soldering.

4. PCB Assembly Options: Single vs Double-Sided, Manual, and Turnkey Approaches

Single-Sided vs Double-Sided SMT: Double-sided requires more than one reflow cycle, and must be thermally balanced.

Manual Assembly: Still used for prototypes or specialized low-volume boards.

Turnkey vs Consignment PCB Assembly:

• Turnkey PCB Assembly→ Manufacturer sources all parts.
• Consignment PCB Assembly→ Customer supplies parts; manufacturer only assembles.

Also Read: How to Consign Parts to JLCPCB

Turnkey vs Consignment PCB Assembly

Main PCB Soldering Techniques

1. Reflow Soldering

Reflow soldering is the primary method for soldering SMT components. The reflow soldering process includes applying solder paste via a stencil, accurately picking and placing components, and passing the assembled PCB through a reflow oven with multiple thermal zones.

A lead-free solder reflow profile, such as for SAC305 alloy, typically consists of four stages: preheat (120–150 °C), soak (150–180 °C), peak (230–250 °C), and controlled cooling. Mechanical integrity depends on proper intermetallic compound (IMC) formation, while poor thermal management can cause common reflow defects such as tombstoning, voids, or popcorning.

Reflow Soldering Process

2. Wave Soldering

Wave soldering is used for through-hole (THT) components and larger surface-mount (SMT) components; a PCB board is pre-fluxed and passed across molten solder (approximately 250 degrees Celsius).

Wave soldering creates strong solder joints; it is ideal for high pin-count connectors or dual-in-line package integrated circuits (DIP ICs).

Tip: Wave soldering is not recommended for small/pitch SMT components or SMT components due to potential risks of bridging and thermal stresses.

Wave soldering Process

3. Selective Soldering

Selective soldering applies solder in a localized area (pin) using a computer numeric controlled (CNC) soldering nozzle, which can be ideal for hybrid assemblies, i.e., an SMT component that has already been reflowed. By using selective soldering, you do not risk damaging the existing connections and minimize thermal stress on the adjacent components. Selective soldering is slower than wave soldering, but a stronger joint can be achieved.

Selective Soldering Process

4. Hand Soldering

Manual soldering is mostly used for prototyping, rework, and low-volume boards. Hand soldering is flexible and labor-intensive, but errors can and will occur. For example, cold joints and inconsistent wetting. Hence, for high-volume productions, hand soldering should not be recommended.

Conclusion

Mastering PCB assembly is about precision, consistency, and quality control—from selecting the right assembly method (SMT, THT, or mixed) to applying the proper soldering technique for each component type. Understanding how to prevent and fix assembly defects not only improves circuit borad reliability but also reduces production waste and rework.

When your project demands industrial accuracy, faster turnaround, and consistent solder joints, partnering with a professional PCBA manufacturer like JLCPCB SMT Assembly Service ensures every step—from stencil printing to inspection—is optimized for high yield and performance. Whether you’re building prototypes or batch products, JLCPCB helps turn your PCB designs into production-ready boards with efficiency and confidence.