Assembly Options#

Having a bare PCB and a bag of components is only half the job — the components still need to get onto the board with reliable solder joints. The assembly method depends on the component mix (through-hole, SMD, fine-pitch), the quantity (one board or fifty), available equipment, and turnaround requirements. Choosing the wrong assembly method leads to either unreliable connections or wasted time.

Hand Soldering#

Hand soldering with an iron is the most accessible assembly method. It is what most people learn first, and it is entirely adequate for a wide range of prototype work.

What hand soldering handles well:

• Through-hole components of any size
• SMD passives down to 0603 (0805 is comfortable; 0402 is possible but taxing)
• SMD ICs in SOIC, SSOP, and TQFP packages (with flux and fine solder or solder wick for bridging)
• Connectors, switches, and mechanical components

What hand soldering struggles with:

• QFN packages — the thermal pad is underneath the component and cannot be reached by an iron
• BGA packages — solder balls under the component are completely inaccessible
• 0201 and smaller passives — too small to see, place, or solder reliably without magnification and specialized tools
• Large quantities — hand soldering one board is fine; hand soldering twenty identical boards is soul-crushing and error-prone

For hand soldering, good equipment matters more than technique: a temperature-controlled station (not a cheap fixed-temperature iron), fine-point tips (conical or chisel), good flux (liquid flux pen or flux paste), and thin solder wire (0.5 mm or 0.3 mm for fine-pitch work). Proper magnification — a stereo microscope or at minimum a headband magnifier — transforms the experience once work moves below 0805 parts.

Hot Air Rework#

A hot air rework station blows temperature-controlled air through a nozzle to heat components from above. It is the second essential tool for prototype assembly, especially for components that hand soldering cannot reach.

Primary uses:

• QFN soldering: apply solder paste to the pads, place the component, and reflow with hot air. The thermal pad underneath gets soldered through the heat conducted through the package.
• Removing components for rework: hot air heats all the joints simultaneously, allowing the component to be lifted off.
• Small-batch SMD assembly: with solder paste on the pads and components placed manually, hot air can reflow one component at a time.

Guidance for hot air work:

• Use the largest nozzle that fits over the target component. Larger nozzles provide more even heating.
• Protect nearby components with Kapton tape or aluminum foil if they are heat-sensitive.
• Avoid rushing. Give the board time to preheat gradually — thermal shock cracks ceramic capacitors.
• Watch the solder, not the timer. Solder paste changes from gray and matte to shiny and liquid at the moment of reflow. That visual transition confirms the component has self-aligned and reflow is complete.

Reflow Oven (or Skillet)#

For boards with many SMD components, reflowing the entire board at once is far more efficient and reliable than soldering components individually.

The process:

1. Apply solder paste to all SMD pads using a stencil.
2. Place all components onto the paste (manually or with a pick-and-place machine).
3. Run the board through a temperature profile that preheats, reflows, and cools the solder.

Equipment options:

• Purpose-built reflow ovens (desktop models from $200-$2000) offer controlled temperature profiles with top and bottom heating. These are the most reliable option.
• Modified toaster ovens with aftermarket controllers work surprisingly well for prototyping. The key is a controller that can follow a proper reflow profile (preheat, soak, reflow, cooling).
• Hot plates and skillets provide bottom-only heating, which works for single-sided boards. Less even than an oven, but adequate for prototyping.
• Hot air can reflow an entire small board when applied slowly across the whole board area.

The stencil is critical. A well-cut stainless steel stencil (available from the same companies that make PCBs — often for $5-$15 with a board order) deposits precise solder paste volumes on each pad. Without a stencil, the alternatives are dispensing paste from a syringe (slow and inconsistent) or spreading it by hand (very inconsistent).

Pick-and-Place Services#

For more than a handful of boards, or for boards with components that are difficult to solder by hand (fine-pitch BGA, 0201 passives, large QFN arrays), outsourcing assembly to a PCB assembly house is often the best option.

Common services:

• JLCPCB — Low-cost assembly from their parts inventory. Fast turnaround, limited component selection (their in-stock parts only, though extended libraries are improving). Best for designs using common jellybean parts.
• PCBWay — Broader component sourcing options, including customer-supplied parts. More flexibility but higher cost and longer turnaround.
• MacroFab, Tempo Automation, Advanced Assembly — North American services with faster turnaround, better communication, and higher prices. Better for complex or time-critical builds.
• Local assembly houses — Worth finding for fast iteration. Being able to visit the factory, review the first board, and make adjustments is invaluable.

Required deliverables:

• Gerber files (or the native EDA files)
• Bill of materials (BOM) with manufacturer part numbers, quantities, and reference designators
• Centroid file (also called pick-and-place file or XY data): the X, Y position and rotation of every component
• Assembly drawing showing which side each component goes on and any special instructions

Most EDA tools generate these files automatically, but they should be verified before submitting. A wrong rotation in the centroid file puts every instance of that component on backwards.

Stencils#

The solder paste stencil bridges the gap between the PCB and the assembly process:

• Laser-cut stainless steel stencils are the standard. They are thin (0.1-0.15 mm), precise, and reusable for hundreds of boards. Ordering them alongside the PCBs is common — most fabricators offer them as an add-on.
• Kapton (polyimide) stencils can be cut with a laser cutter or craft cutter. They are cheaper but less precise and wear out faster. Adequate for prototyping.
• Hand-cut stencils are possible for large-pad designs but impractical for fine-pitch work.
• Frameless vs framed: Frameless stencils are cheaper and easier to store. Framed stencils align better in a stencil printer. For hand-use, frameless with tape-hinge alignment is the common prototype approach.

Using a stencil well takes practice. The key variables are: squeegee pressure (firm and consistent), squeegee angle (about 45 degrees), paste viscosity (should be smooth, not dried out), and alignment (paste should land on the pads, not beside them).

Mixed Assembly#

Most real boards use mixed assembly: SMD components reflowed with paste, then through-hole components soldered by hand or by wave/selective soldering.

Typical mixed assembly flow:

1. Print solder paste (stencil), place SMD components, reflow.
2. Hand-solder through-hole components (connectors, large capacitors, transformers).
3. Inspect and touch up.

This is why through-hole components on an otherwise SMD board add process complexity — they require a separate manual step. Finding SMD equivalents for through-hole parts keeps the assembly cleaner. But some components (large connectors, high-current inductors, some electrolytic capacitors) are only available or practical in through-hole form.

Cost and Turnaround Tradeoffs#

The assembly method is a business decision as much as a technical one:

For a single prototype, hand assembly is almost always faster in wall-clock time, even if it takes longer to solder. For 10+ identical boards, an assembly service is almost always faster and more reliable. The crossover point is somewhere around 3-5 boards for most designs.

Tips#

• Order stencils alongside every PCB order — the marginal cost is small and the improvement in paste consistency is significant
• Keep solder paste refrigerated and track its expiration date; dried-out or expired paste causes most reflow defects
• For mixed-assembly boards, reflow SMD components first, then hand-solder through-hole parts — reversing the order risks reflowing existing joints
• Photograph the board after assembly and before power-on as a reference baseline for any future rework or failure analysis

Caveats#

• Solder paste expires. Solder paste has a shelf life (typically 6 months refrigerated) and must be stored properly — old paste causes poor wetting, solder balls, and unreliable joints
• Reflow profiles matter. Too fast and the result is tombstoning and solder balls; too hot and components get damaged; too slow and wetting is poor — follow the solder paste manufacturer’s recommended profile
• Assembly services have component constraints. JLCPCB’s “basic” parts list is limited — if the design uses parts not in their inventory, “extended” parts cost extra, or components must be supplied separately, adding time and cost
• First-article inspection saves money. When using an assembly service for the first time, requesting a first-article inspection means one board is built and photographed before assembling the full batch — catching a rotated IC on one board is cheaper than finding it on fifty
• ESD precautions apply during hand assembly. An ESD wrist strap and grounded mat cost $20; replacing a static-damaged IC on a finished board costs hours — ESD protection is essential, especially when handling CMOS devices and sensitive analog parts
• Clean the flux. Flux residue is conductive when wet and can be corrosive over time — for any board that needs to be reliable, cleaning the flux after soldering with isopropyl alcohol and a brush is adequate for most no-clean fluxes, despite the name