Getting a printed circuit board fabricated is only half the story. Once the bare board comes back from the manufacturer, it still needs to have all its components attached before it actually does anything useful. That process is called PCB assembly, and the method used to assemble a board has a significant impact on cost, lead time, and the kind of components your design can use.
If you are new to hardware development, or if you are moving from breadboard prototyping to actual PCB production for the first time, understanding the main assembly methods helps you make better design decisions and have more informed conversations with your manufacturing partners. Here is a plain-English breakdown of how PCB assembly actually works.
The Two Main Component Types Drive Everything
Before getting into assembly methods, it helps to understand that there are two fundamentally different types of electronic components, and the way they attach to a board determines how they get assembled.
Through-hole components have wire leads that pass through drilled holes in the board and are soldered on the opposite side. They are larger, easier to handle, and mechanically very robust. If you have ever looked at an older piece of electronics and seen components with legs poking through the board, those are through-hole parts.
Surface mount components, also called SMD or SMT components, sit directly on pads on the board surface with no holes required. They can be extremely small, much smaller than through-hole equivalents, and they can be placed on both sides of the board. The vast majority of modern electronics use surface mount components because they are smaller, lighter, and much faster to place using automated equipment.
Most real-world boards use a mix of both types, with surface mount components handling the bulk of the circuitry and through-hole reserved for connectors, large capacitors, and anything that needs extra mechanical strength.
Surface Mount Assembly: How It Works
Surface mount assembly, or SMT assembly, is the dominant method for modern PCB production and the one used by most contract manufacturers and PCB assembly services today.
The process starts with solder paste, a gray, slightly sticky mixture of tiny solder particles suspended in flux. A stencil, which is a thin metal sheet with openings cut precisely to match the pad layout of the board, is placed over the board. Solder paste is applied over the stencil and squeegeed across it so that paste is deposited only on the exposed pads through the openings. The stencil is then removed, leaving precise deposits of solder paste on every pad.
Next, a pick-and-place machine uses vacuum nozzles and a vision system to pick components from reels and trays and place them precisely onto the solder paste deposits. Modern pick-and-place machines are fast and accurate, capable of placing thousands of components per hour with placement accuracy measured in thousandths of an inch.
The populated board then goes into a reflow oven, which heats the board through a carefully controlled temperature profile. The flux in the solder paste activates and cleans the surfaces. The solder particles melt and flow together, wetting the component leads and the pads to form the solder joints. The board cools and the solder solidifies, creating permanent electrical and mechanical connections.
The result is a fully assembled board that came together without a human hand touching most of the components. For production volumes, this process is highly consistent, fast, and cost-effective.
Through-Hole Assembly: How It Works
Through-hole components are assembled by inserting their leads through drilled holes in the board and then soldering them. There are two main ways that soldering happens.
Wave soldering is the traditional production method for through-hole assembly. Boards are passed over a wave of molten solder that contacts the underside of the board, wetting the exposed lead tips and forming solder joints at all the through-hole connections simultaneously. It is fast and well-suited to boards with a consistent layout of through-hole components.
Selective soldering is a more targeted approach where a precise nozzle applies molten solder only to specific areas of the board. It is used when the board has a mix of components that makes wave soldering impractical, or when some components cannot tolerate the heat of the full wave soldering process.
Hand soldering is exactly what it sounds like. A skilled technician uses a soldering iron to individually solder each joint. For prototypes, low-volume production, and components that cannot go through automated processes, hand soldering is still widely used and can produce very high quality results when done by an experienced assembler.
Mixed Assembly: Handling Both Component Types
Most production boards use both surface mount and through-hole components, which means the assembly process needs to handle both. The typical approach is to run the surface mount assembly process first, getting all the SMD components reflowed onto the board, and then handle the through-hole components afterward through wave soldering, selective soldering, or hand soldering depending on the design and volumes involved.
This sequencing matters when you are designing a board. Where you place through-hole components relative to surface mount components on the bottom side of the board affects whether wave soldering is practical and what masking may be required to protect already-assembled SMD components during the wave solder step.
For most designers, the practical implication is to be aware that mixing component types adds process steps and therefore cost and lead time compared to a purely surface mount design. Where a through-hole component can be replaced with a surface mount equivalent without compromising the design, doing so simplifies assembly.
Turnkey vs. Consigned Assembly
When you are working with a PCB assembly service, you will encounter two main service models that are worth understanding before you request a quote.
Turnkey assembly means the assembly house sources all the components, fabricates or procures the bare boards, and delivers fully assembled boards. You provide the design files, the bill of materials, and the pick-and-place file, and the assembler handles everything else. It is the most convenient option and is particularly well-suited to prototype and low-volume production where managing component procurement yourself is not efficient.
Consigned assembly, sometimes called partial turnkey, means you supply some or all of the components yourself and the assembler provides the labor and process. This approach gives you more control over component sourcing, which matters when specific approved vendors are required, when you have existing component inventory, or when certain parts are difficult for an assembler to source on your behalf.
Most online PCB and assembly services today default to turnkey because it is simpler for customers and more profitable for the assembler. Understanding the distinction helps you ask the right questions when cost or component control is a priority.
What Files You Need for Assembly
Getting a board assembled requires more than just the Gerber files used for fabrication. The assembly process needs additional information about what components go where and in what orientation.
The bill of materials, or BOM, lists every component on the board with its reference designator, description, value, and manufacturer part number. A clean, accurate BOM is essential for the assembler to source and verify components correctly. Errors in the BOM are one of the most common causes of assembly problems.
The pick-and-place file, also called the centroid file or component placement file, lists every component’s location on the board in X and Y coordinates along with its rotation angle. The pick-and-place machine uses this file to know exactly where to put each component. An error in this file puts components in wrong locations or wrong orientations, which creates functional problems that may not be immediately obvious.
Assembly drawings or notes communicate anything that is not captured in the BOM or pick-and-place file, such as specific orientation requirements for components that the pick-and-place file rotation alone does not fully specify, conformal coating requirements, or any components that need special handling.
Choosing an Assembly Partner
For prototype and low-volume production, the online integrated PCB fabrication and assembly services have made getting assembled boards remarkably accessible. You upload your files, specify your requirements, and receive assembled boards without managing multiple vendors or logistics.
For higher volumes or more demanding quality requirements, dedicated contract manufacturers offer a higher level of process control, testing capability, and quality management than online prototype services. The relationship with a contract manufacturer is more involved but delivers more capability and more accountability at production scale.
Working with manufacturers who have demonstrated assembly experience relevant to your design complexity is worth the effort of verification. A manufacturer like Avanti Circuits that combines PCB fabrication expertise with assembly capability offers the advantage of a single point of accountability for both the bare board and the finished assembly, which simplifies communication and reduces the risk of issues that fall between two separate vendors.
The Bottom Line
PCB assembly is where a designed circuit becomes a working product, and understanding the main methods helps you design better boards, communicate more effectively with your manufacturing partners, and make more informed decisions about where to have your boards assembled.
Surface mount assembly handles the majority of modern component types with speed and consistency. Through-hole assembly remains essential for specific component categories. Mixed designs use both in sequence. Turnkey services handle the whole process for you while consigned assembly gives you more control over components. And the files you prepare, the BOM and pick-and-place file in particular, are as important to assembly quality as the Gerber files are to fabrication quality.
Get these fundamentals right and the path from a finished design to a working assembled board is a lot more predictable.
