· By ZDCL Engineering Team
PCB assembly (PCBA) transforms a bare printed circuit board into a functional electronic device by mounting and soldering components. Understanding the different assembly technologies — and how to prepare your design files — can significantly reduce costs, improve quality, and accelerate production timelines. This guide covers everything you need to know.
Surface Mount Technology (SMT)
SMT is the dominant PCB assembly method, accounting for the vast majority of components on modern circuit boards. Surface mount components have small metal tabs or terminations that are soldered directly onto pads on the PCB surface — no drilled holes required.
How it works: Solder paste (a mixture of tiny solder spheres and flux) is applied to the PCB pads using a stainless steel stencil and squeegee. A pick-and-place machine then precisely positions each component onto the paste deposits. The assembled board passes through a reflow oven where the solder paste melts, forming permanent electrical and mechanical connections.
Advantages: High component density, excellent high-frequency performance (shorter lead lengths = lower parasitic inductance), faster assembly speeds (thousands of components per hour), and smaller overall product size. SMT components are also generally cheaper than through-hole equivalents.
ZDCL capability: Our high-speed SMT lines feature automated solder paste inspection (SPI) before placement and automated optical inspection (AOI) after reflow, ensuring first-pass yields above 99.5%.
Through-Hole Technology (THT)
Through-hole assembly involves inserting component leads through drilled holes in the PCB and soldering them to pads on the opposite side. While largely replaced by SMT for high-volume production, through-hole remains essential for certain applications.
When to use through-hole: High-power components (transformers, large capacitors, power transistors), connectors that experience mechanical stress, and applications requiring the strongest possible mechanical bond. Through-hole solder joints are mechanically stronger than SMT joints.
Soldering methods: Wave soldering (the board passes over a wave of molten solder) for mixed-technology boards. Selective soldering uses a programmable nozzle to solder specific through-hole components without affecting nearby SMT parts. Hand soldering is used for prototypes and low-volume specialty assemblies.
Mixed-Technology Assembly
Most real-world PCB designs use a combination of SMT and through-hole components. The typical assembly sequence is: SMT components on the top side → reflow → through-hole components inserted → wave or selective soldering → SMT on the bottom side (if required) → second reflow.
For mixed-technology boards, design the layout so through-hole components are grouped together on one side of the board whenever possible. This minimizes the number of soldering steps and reduces cost.
BGA and Fine-Pitch Assembly
Ball Grid Array (BGA) packages and fine-pitch components (QFN, LGA, 0.4mm pitch and below) require specialized assembly techniques. The solder balls under a BGA package are not accessible for visual inspection after reflow, so X-ray inspection is essential to verify proper solder joint formation.
ZDCL uses automatic optical inspection (AOI) for visible joints and X-ray inspection for BGA devices to ensure every solder connection — visible or hidden — meets IPC-A-610 Class 2 or Class 3 acceptance criteria.
Preparing Your Files for PCBA
To ensure a smooth assembly run, provide your PCBA manufacturer with the following deliverables:
- Bill of Materials (BOM): A complete list of every component including manufacturer part number, quantity, reference designator, and any acceptable substitutes. Use a spreadsheet format (Excel or CSV) with clear column headers.
- Gerber Files: The same Gerber package used for bare PCB fabrication — copper layers, solder mask, silkscreen, and drill files.
- Centroid / Pick-and-Place File: Also called a placement file or XY data file. This ASCII text file contains the X/Y coordinates, rotation angle, and package type for every surface mount component. Export from your CAD tool in CSV format.
- Assembly Drawings: PDF drawings showing component locations, orientations, and any special assembly notes. Include polarity indicators and pin-1 markings clearly.
- Test Specifications: If you require in-circuit testing (ICT), flying probe testing, or functional testing, provide the test specifications, test point locations, and pass/fail criteria.
Conformal Coating and Box-Build
For harsh-environment applications, conformal coating (acrylic, silicone, polyurethane, or parylene) protects assembled PCBs from moisture, dust, chemicals, and temperature extremes. ZDCL offers selective conformal coating using automated spraying equipment, ensuring coverage on critical areas while keeping connectors and test points clear.
For customers who need a complete turnkey solution, we also provide box-build assembly — integrating the assembled PCB into its final enclosure with wiring, connectors, and mechanical assembly.
Why Combine PCB Fab and Assembly with One Supplier?
Using separate suppliers for PCB fabrication and assembly introduces coordination overhead, shipping delays, and finger-pointing when issues arise. ZDCL provides PCB fabrication and PCBA assembly under one roof in Shenzhen, eliminating these logistics challenges. Our single point of accountability and integrated quality control ensure faster turnaround and consistent quality from bare board to assembled product.
Ready to Start Your PCBA Project?
Send us your BOM, Gerber files, and placement data for a complete PCBA quotation — most quotes are returned within 2 hours.
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