SMD Soldering Guide

This guide is meant to help in the soldering of SMD components on PCBs. A single board could be composed of two side, with only one being SMD. This guide is not meant to help in the soldering of boards with SMD components on both sides, as for such processes a soldering oven is needed.

Before Soldering

PCB

Optimal PCB design

For correct soldering, the board design must be optimal, following the manufacturer’s guidelines. Here are some examples of common errors:

• Incorrect: solder mask the same size as pads, wrong pad size, traces under the footprint.
• Incorrect: traces and vias under the footprint, traces entering pads not aligned parallelly.
• Correct: clean designs with proper pad size and trace routing.

Solder paste prepping

There are multiple guidelines to use when selecting the proper solder paste, but they are not the focus of these guide.

Once a solder paste is selected it is advisable to keep it refrigerated, as per datasheet recommendation.

Before starting the soldering process, allow the solder paste to reach room temperature if it has been stored in a refrigerator. Then, vigorously mix the solder paste for a few minutes. This ensures proper distribution of the flux and improves its spreadability during application. The paste should be uniform and easily malleable to pass effectively through the stencil.

If the paste appears excessively dry due to improper storage, it is possible, as a last resort, to re-plasticize the flux resin by adding a few drops of a specific solvent (e.g.,you can replistify ROL0 resin with a 70% ethanol, 30% white spirit solution). This should be considered a temporary fix while waiting to obtain fresh solder paste.

Board and stencil prepping

It is advisable to clean both the board and the stencil with isopropil alchool (>90°). to remove any contaminants. This step is essential to ensure a clean and durable solder joint.

Stencil pre-heating

Insert the PCB into the adapter to align it with the stencil. Evenly preheat the PCB and stencil using a hot air station (approx. 100°C). Be careful not to deform the stencil with excessive heat.

Stencil application

Apply the solder paste in two or three spots along the long side of the board. Use a spatula to spread it parallel to the short side, moving left to right and vice versa.

Visually inspect the application: the paste should be confined within the pads, without connecting them. Repeat if necessary.

Component placement

Using SMD tweezers and a KiCad plugin (HTML Bom) , place the components following the positions and quantities specified in the BOM. Mark off the components as you place them to avoid mistakes.

Reflow Curve

Extracting the Curve from the Datasheet

Analyze the solder paste datasheet to identify the optimal reflow curve and adapt the parameters to your available heat plate.

Programming the Heat Plate

Configure the heat plate according to the extracted parameters and save the settings.

Test Run Without Board

Perform a test run without the board to verify the thermal behavior and parameter accuracy.

First Reflow (Sensors Only)

Place the board on the heat plate, centering the sensors for uniform heating. Start the first reflow cycle.

Important: When the solder is freshly melted and still liquid, you can gently move the sensors with tweezers to help proper alignment. This self-aligning effect disappears once the flux evaporates, so adjustments must be done early.

Alignment Verification and Pull Test

After natural cooling (no forced cooling), visually and mechanically check the sensor soldering by performing a light pull test.

Second Reflow (Remaining Components)

Place the board back on the heat plate and proceed with the second reflow cycle to solder the remaining components.

Fisnishing

Manual Soldering on the Opposite Side

Manually solder the components on the opposite side of the board using a fine-tip soldering iron.

Soldering USB Anchors

Finally, solder the USB anchors to ensure mechanical stability and good electrical conduction.