Soldering method in component mounting process

What Is Electronic Component Assembly? Surface Mount vs. Through-Hole Mounting

In electronics manufacturing, component assembly means mounting electronic components such as semiconductor devices and ICs onto printed circuit boards (PCBs) and forming the electrical connections required for the assembly to function. The board, components, joining method, and assembly process must be selected according to the electrical, mechanical, thermal, reliability, and production requirements of the product.

Component mounting processes can be broadly divided into surface-mount and through-hole mounting processes, which commonly use different soldering methods.

Surface-Mount Process

Surface-mount technology (SMT) mounts the terminals or leads of surface-mount devices onto connection pads on the surface of a PCB. Reflow soldering is widely used for this process.

Through-Hole Mounting Process

In through-hole mounting, component leads are inserted into plated through-holes in the PCB and soldered to the surrounding lands and hole metallization. Wave or other flow soldering methods are commonly used in volume production.

Advantages and Limitations of Surface-Mount and Through-Hole Assembly

Surface-mount and through-hole assembly each have characteristics that make them suitable for different components and product requirements.

Surface Mount

Advantages $D83D$DE42 Supports small electronic components and high-density circuit layouts. $D83D$DE42 Well suited to automated high-volume assembly. Limitations $D83D$DE41 Fine-pitch assemblies can be more difficult to repair manually. $D83D$DE41 Joint mechanical performance depends strongly on package, pad, solder-joint geometry, board design, and application conditions. $D83D$DE41 Reflow exposes the PCB and mounted components to a controlled thermal profile, so component and board thermal limits must be considered.

Through-Hole Mount

Advantages $D83D$DE42 Leads pass through the PCB and can provide robust mechanical retention for suitable component designs. $D83D$DE42 Useful for large, mechanically loaded, or specialized components that are not suited to standard SMT placement. Limitations $D83D$DE41 Some connectors, large capacitors, and irregular components may require dedicated insertion equipment or manual insertion. $D83D$DE41 Through-hole components and required board area can limit mounting density compared with fine-pitch SMT.

Reflow Soldering Process for Surface-Mount Technology (SMT)

A representative SMT reflow process is shown below.

1. Print solder paste onto the PCB

2. Place components

3. Reflow solder

4. Flux cleaning when required

Solder Paste Printing

Solder can be supplied by several methods, but stencil printing of solder paste is widely used for SMT assembly. Printing is a critical process because solder paste deposit position and volume influence downstream solder-joint formation. Solder paste inspection (SPI) may be used to check deposit volume, height, area, or position depending on process requirements. Maintaining stencil cleanliness is also important for stable printing performance.

Learn more about solder paste printing and metal stencil cleaning:
	PCB Assembly Column Metal Stencil Printing Process and Common Problems Explains metal stencils, the solder paste printing process, common printing problems, and stencil cleaning.
	Product Information Metal Stencil / Screen Mask Cleaning Cleaning solutions for printing materials including solder paste, thick-film paste, conductive adhesives, resist inks, and temporary-fixing adhesives.

Component Placement

A pick-and-place machine uses placement nozzles to pick electronic components from tape-and-reel feeders, trays, or other feeders and place them at the specified positions on the PCB.

Reflow Soldering

A reflow oven heats the PCB assembly according to a controlled temperature profile so that the solder paste reaches the conditions required to form solder joints. Production reflow systems commonly use multiple heating and cooling zones to control thermal history through the process.

Example of a reflow temperature profile

Preheat / Soak The assembly temperature rises in a controlled manner while volatile paste components begin to evaporate and the flux becomes active according to the solder paste design. Reflow / Peak Heating The solder joints are heated above the solder alloy liquidus for the required time while remaining within component and PCB thermal limits. Cooling The assembly is cooled so the molten solder solidifies and the joints are formed.

A reflow profile must satisfy the process window for the solder paste, PCB, and components. Heating that is too rapid or too severe can increase the risk of process defects or component damage, while insufficient heating can prevent proper solder-joint formation. The appropriate profile should therefore be established from the solder paste supplier's recommendations, component thermal limits, board characteristics, oven capability, and actual measured board temperatures.

Reflow heating technologies include convection, infrared, and vapor-phase approaches. Equipment design varies by process requirements.

Flux volatiles and residues can accumulate on reflow-oven interiors and parts, so periodic maintenance and cleaning may be required.

	Product Information KakenClean DF-99U Reflow Oven Cleaner DF-99 is a water-based cleaner for removing flux contamination from reflow ovens and related parts. It can also be used for selected flux-contaminated equipment parts, subject to compatibility and use conditions.

Flux Cleaning

After reflow soldering, flux residue may be cleaned from the PCB when required by the product or process. The need for cleaning depends on the flux type, residue characteristics, electrical and reliability requirements, downstream processes, and cleanliness specifications. When selecting a cleaning process, consider cleaning performance, environmental and worker-safety requirements, compatibility with PCBs and components, waste handling, maintenance, and operating cost.

	Technical Solutions What Is Flux Cleaning? Explains why flux residue is cleaned, representative cleaning methods, and considerations for effective cleaning.

Flow Soldering Process for Through-Hole Assembly

A representative flow soldering process for through-hole assembly is shown below.

1. Insert components

2. Apply flux

3. Flow solder

4. Flux cleaning when required

Component Insertion

Component leads are formed as required for the PCB hole pattern and inserted into through-holes using automatic insertion equipment where appropriate. Components that are unsuitable for automatic insertion may be inserted manually.

Flux Application

Flux can be applied by methods such as dipping, foam fluxing, or spraying. The appropriate method depends on the assembly, equipment, flux design, required coating amount, and production conditions. Spray fluxing is widely used in modern flow and selective soldering equipment because application can be controlled locally and consistently.

	PCB Assembly Column What Is Soldering Flux? Explains the role, composition, and types of flux used in soldering.

Flow / Wave Soldering

The underside of the PCB is brought into controlled contact with molten solder to form solder joints at through-hole leads and other exposed solderable areas. Wave-soldering equipment uses one or more solder waves depending on equipment design and process requirements.

Wave geometry, conveyor conditions, flux application, preheating, solder temperature, and contact time are controlled to achieve adequate solder wetting and hole fill for the applicable assembly design.

Flux Cleaning

Through-hole assemblies may contain components with limited resistance to cleaning liquids or immersion processes. Where full-board cleaning is unsuitable, localized cleaning may be used for the soldered area. Cleaner compatibility, worker exposure, odor, cleaning tools, and the risk of leaving fibers or brush debris should be considered.

Thumbnail for the flux cleaning article" width="100" height="100">	Technical Solutions What Is Flux Cleaning? Explains representative flux cleaning methods and key considerations for selecting a cleaning process.

Mixed Reflow and Flow Assembly and the Role of Solder Pallets

Some PCB assemblies combine SMT reflow soldering with through-hole flow soldering. A representative sequence is shown below.

1. Print solder paste	2. Place components	3. Reflow solder
4. Turn over PCB	5. Install solder pallet	6. Insert components
7. Apply flux	8. Flow solder	9. Clean if required

Solder Pallets

In mixed assembly, a heat-resistant solder pallet, also called a wave pallet or carrier pallet, can be used during the flow soldering process to expose only the areas that need contact with solder.

	Protect selected components and board areas from the solder wave and process heat.
	The pallet masks areas that should not be exposed directly to molten solder during the flow process.
	Restrict solder contact to required through-hole soldering areas.
	Openings in the pallet expose the through-hole lead areas while shielding previously assembled SMT areas from direct solder contact.
Flux and solder residue can accumulate around pallet openings and edges. Regular cleaning helps maintain dimensional condition and reduce unwanted solder or contamination transfer.

	Product Information Solder Pallet Cleaning Systems and Cleaners KAKEN TECH offers cleaning systems and cleaning agents designed for removing flux and solder-process contamination from solder pallets and carrier pallets.

When Manual Soldering Is Used and the Basic Process

Although electronics assembly is highly automated, manual soldering with an electric soldering iron is still used for applications such as rework, repair, engineering changes, prototypes, and installation of components unsuitable for automated reflow or flow processes.

	Component rework and repair
		Manual soldering can be used to correct unacceptable solder joints or replace failed or changed components.
	Post-assembly component installation
		Components may be installed after the main assembly process when their thermal, solvent, mechanical, or process limitations make the standard reflow or flow process unsuitable.

Basic Manual Soldering Process

A representative manual soldering process includes surface preparation, heating, solder application, and cleaning when required.
1. Surface Preparation	Remove contamination that could interfere with solder wetting using a cleaning method compatible with the materials and process.
2. Heating	Set and control the soldering iron tip temperature according to the solder alloy, joint thermal mass, component limits, and process requirements.
3. Solder Application	Position the iron and supply the appropriate amount of solder so the joint surfaces reach the required soldering condition and the solder wets correctly.
4. Cleaning	If cleaning is required, remove applicable flux residue using a method that is compatible with the assembly. Localized cleaning with swabs, brushes, or spray cleaners may be used depending on product requirements.
Manual soldering directly affects solder-joint quality and should be performed to the applicable workmanship and product requirements. The JIS C 61191 series specifies requirements for printed board soldered assemblies; JIS C 61191-1:2021 covers general requirements for soldered assemblies using surface-mount and related technologies, while JIS C 61191-3 covers through-hole mount soldered assemblies. JIS C 61191-1:2021 citeturn69search93turn69search94 JIS C 61191-3:2020 citeturn69search92

Reference

Standard Micro-Soldering Technology, 3rd Edition
Edited by the Micro Soldering Education Committee, Japan Welding Engineering Society
The Nikkan Kogyo Shimbun, Ltd.