The Ultimate PCB Assembly Guide for Original Equipment Manufacturers

Printed circuit board (PCB) assembly involves mounting electronic components onto a bare PCB to create a complete, functional electronic circuit. This process encompasses component placement, soldering, inspection and final testing, transforming a blank board into the base for virtually all modern electronic devices.

PCB assembly can be performed using two main methods: surface-mount technology and through-hole technology.

Surface-mount technology (SMT) involves placing components, known as surface-mount devices (SMDs), directly onto PCB pads coated with solder paste, followed by a controlled thermal reflow to create solder joints. In contrast, through-hole technology (THT) requires component leads to be inserted into pre-drilled holes on the PCB and soldered on the opposite side, forming robust mechanical and electrical connections.

In this guide, you’ll learn when and why to choose SMT versus THT, as well as how specialised PCB assembly services — including potting and encapsulation, conformal coating and low-pressure overmoulding — enable you to extend product life in harsh environments.

By the end, you’ll also know how an electronic contract manufacturing partner can help you with PCB assembly services and what to look for in an electronics manufacturing services provider.

What is surface-mount technology?

Surface-mount technology has modernised electronics manufacturing by enabling the attachment of components directly to the PCB surface without the need for drilled holes. SMT components are typically smaller than their through-hole counterparts and use metal terminations — tabs or end caps — instead of wire leads to form soldered connections.

The assembly process begins with stencil-printed solder paste on PCB pads, followed by automated pick-and-place machines accurately depositing each SMD and a reflow oven soldering them in a controlled thermal profile.

Although through-hole technology predates SMT, this method remains indispensable where mechanical strength is paramount. In THT, component leads pass through plated holes drilled into the PCB, and solder is applied on the opposite side, forming strong, reliable joints. This method is often favoured for connectors, large capacitors and components subjected to vibration or high mechanical loads.

Surface-mount technology vs. through-hole technology

SMT’s greatest strengths lie in miniaturisation and high-volume throughput. By supporting double-sided placement and extremely fine-pitch components, SMT maximises board space and enables the compact, lightweight designs common in consumer electronics.

Automated placement and reflow processes reduce manual labour, lower per-unit costs in large runs and improve consistency. However, the initial capital expenditure for pick-and-place equipment, reflow ovens and inspection systems can be substantial. Furthermore, the small solder joints of surface-mount technology can be less mechanically robust under high-stress conditions without additional reinforcement.

THT’s primary advantage is its mechanical sturdiness. Components mounted via through-hole technology can withstand substantial axial and radial forces, making THT ideal for applications where reliability under stress is critical.

The method also simplifies prototyping and manual rework, as leads are easily accessible. On the downside, THT consumes more board area per component, limits routing density and involves slower, more labour-intensive insertion and soldering processes, resulting in higher per-board costs for large volumes.

Comparing SMT and THT: pros, cons and suitable applications

Key considerations for quality PCB assembly

Ensuring consistently reliable PCB assembly requires more than just accurate component placement — it demands a comprehensive approach that begins at the design desk and carries through every step of production and testing.

There are four critical pillars that ensure quality control in PCB assembly: design for manufacturability (DFM), process controls and inspection, material handling and the assembly environment, and testing and validation.

By integrating these practices into your workflow, you can dramatically reduce rework, accelerate time-to-market and achieve high yields without sacrificing quality.

Design for manufacturability (DFM)

Implementing DFM guidelines early in development reduces cost and accelerates time-to-market.

Critical design for manufacturability checks include verifying pad dimensions, solder mask clearances, component spacing and thermal relief for through-hole pads. Specific DFM tools can automatically flag issues like insufficient annular ring or trace spacing, enabling rapid iteration before fabrication.

If you’re looking to engage a PCB assembly services partner, you should do so early in the design process to ensure your board layout aligns with the selected assembly processes and tolerance capabilities.

Process controls and inspection

Consistent process controls are essential to ensure defect-free PCB assembly:

• Solder paste inspection (SPI) measures paste volume and uniformity to prevent opens and insufficient solder joints.
• Automated optical inspection (AOI) scans for component misalignment, polarity errors and solder bridging.
• For ball grid arrays (BGAs) and hidden joints, X-ray inspection catches voids and tombstoning problems.
• Maintaining precise reflow oven temperature profiles prevents solder voids and tombstoning, which are critical for fine-pitch components.

Material handling and the assembly environment

Material handling is also important. Components and PCB materials must be stored in humidity-controlled environments to avoid moisture uptake, which can cause ‘popcorning’ during reflow. Equally, having an ESD-protected assembly area prevents electrostatic discharge damage to sensitive components.

Regularly carrying out equipment calibration of placement machines and stencil printers further reduces risks of contamination and helps you maintain high placement accuracy and repeatability.

Testing and validation

Post-assembly testing — ranging from visual inspection to in-circuit and functional tests — then validates electrical performance before boards are shipped.

Visual inspections catch obvious defects, while in-circuit testing verifies solder connectivity and component values. Functional testing under simulated operating conditions confirms performance and reliability. Incorporating boundary scan or flying probe tests can further enhance coverage for complex, high-density boards.

The role of electronic contract manufacturing in PCB assembly

Outsourcing your PCB assembly project to a specialist electronic contract manufacturing partner unlocks numerous advantages.

Electronic contract manufacturing providers offer turnkey solutions that encompass component sourcing, PCB assembly, mechanical integration and final system testing. Electronics manufacturing services extend this model by providing after-market support, repair and reverse logistics. This comprehensive approach simplifies your supply chain: instead of coordinating multiple vendors, you benefit from a single point of accountability.

An integrated electronics manufacturing services partner streamlines production workflow, consolidates shipping and maintains unified quality management systems across all stages of the PCB assembly. This approach reduces lead times, lowers per-unit costs and ensures consistent quality across prototype and production volumes.

Leading electronics manufacturing services providers also offer a suite of advanced processes to enhance product performance and longevity.

In demanding applications where your assemblies face moisture, vibration, chemical exposure or mechanical stress, standard PCB assembly alone may not provide the long-term protection you need. That’s where complementary techniques — such as potting and encapsulation, conformal coating, low-pressure overmoulding or overmoulding — come into play.

Potting and encapsulation

Potting is ideal for harsh-environment electronics — such as those in non-automotive commercial transport — where long-term reliability is non-negotiable.

Potting and encapsulation involves filling or coating an assembled PCB with resin (epoxy, silicone or polyurethane) to seal components against moisture, vibration and tampering. Robotic dispensing and meter-mix machines ensure precise compound ratios and void-free coverage.

Conformal coating

A conformal coating applies a thin polymer film (acrylic, silicone, urethane or perylene) over the PCB surface, conforming to component contours and traces. These coatings protect against moisture, dust, chemicals and temperature extremes, extending the service life of electronic devices.

Unlike potting, conformal coatings add minimal weight and thickness, making them suitable for applications where space and weight are constrained, such as medical devices.

Low-pressure overmoulding and overmoulding

Low-pressure overmoulding uses hot-melt polyamides injected at low pressures to encapsulate boards or sub-assemblies within a precision mould, creating a seamless protective barrier without damaging delicate solder joints.

Traditional overmoulding, often at higher pressures, embeds components into thermoplastics or elastomers to form integrated housings or cable assemblies.

Both techniques enhance mechanical robustness and environmental sealing, which is vital for automotive sensors and rugged industrial devices.

Partnering with an electronic contract manufacturing specialist brings expertise, scalability and cost efficiencies. When selecting an electronics manufacturing services provider, look for demonstrated process controls, comprehensive service offerings — from standard PCB assembly services to potting and encapsulation, conformal coating and low-pressure overmoulding — plus strong supply-chain management and certification standards.

Why choose EC Electronics for your PCB assembly services?

As one of the top PCB assembly manufacturers in the UK, EC Electronics has 40 years of experience in assembling and supplying high-quality PCB assemblies to exact specifications.

From basic assemblies to complicated capabilities, such as flexible circuits, thermal substrates and PCBs with blind and buried vias, we can meet all your production needs through our broad range of PCB assembly services — including potting, encapsulation and conformal coating plus low-pressure overmoulding.

Our facilities are equipped with the latest machines, and our services encompass both surface-mount technology and through-hole technology. This means we can offer different levels of service — from supplying PCB prototypes right through to the manufacture of complex, multi-technology printed circuit boards.

At EC Electronics, we use only the best components for the job, sourced from our extensive network of well-known manufacturers and distributors, including Microchip, ST, AVX, VISHAY, Kemet, Bourns, Avnet, Arrow, TTI, Digikey, RS and Farnell. Any moisture-sensitive PCB components are stored in temperature and humidity-controlled chambers to ensure optimal performance.

We work to industry standard IPC-A-610 Class 3, and our quality management system is ATEX certified, meaning all electronics manufacturing processes and products meet the minimum requirements for intrinsic safety in potentially explosive atmospheres. All PCBs we manufacture are also inspected using the latest 3D automatic optical inspection technology to provide assurance of circuit functionality, so you can feel confident your projects are in safe hands with our specialist team.

Whether you’re looking for low or high-volume PCB assembly, we can help and would be happy to talk you through your PCB assembly services requirements.

Contact EC Electronics today at +44 (0)1256 461894 or sales@ecelectronics.com to discuss your next PCB assembly project and obtain a free quote.