How to Reflow

The quickest, easiest, and sometimes only way to solder surface-mount boards is through the process of reflow soldering. This guide is intended to give a rough overview for our in-house process for manufacturing boards.

Prerequisites

Before you start this process, you will need:

• A complete PCB design in an EDA program of your choice (at time of writing, CUSF uses KiCad).
• Sheets of 125um sheets of Mylar.
• All the components you want to solder to the board.
• Solder paste. Make sure to follow Dyson Centre guidelines on acceptable solder.
• Access to the Dyson Center laser cutter. This requires training and explicit permission from Rich to cut Mylar sheets.
• Access to the Dyson Centre IR oven, or suitable alternative. If you are a member of CUSF, speak to Henry Franks for a guide on how to use this.

You may also want a few extra resources for rework:

• A soldering iron.
• Solder wick, multiple sizes are recommended.
• Solder. Again, follow the Dyson Centre guidelines for this.
• Flux pen(s).
• Isopropyl alcohol and dry wipes, for cleaning up.

Getting the Board Made

Boards can be printed in-house by the Design and Technical Services office, previously the Electronics Development Group. Note that boards cannot have silkscreen printed. It is highly recommended to have your board printed with soldermask, as reflow without it is a task fit only for a masochist. If you are with CUSF, speak to Henry again about this. If you are not with CUSF, you can email design-technical-services@eng.cam.ac.uk about this process.

Cutting a Stencil

Cutting a stencil can be done in house easily and at little cost. The steps are as follows (assuming you only want to solder components on the top side of the board):

1. Export your board's front paste layer as a DXF file. Make sure to set your units to mm.
2. If you run into compatbility issues when loading this file into the laser cutter software, you may need to open your file in Solidworks and re-export from there. This will add a watermark to the file, but this won't appear on your cut.
3. Follow the usual procedure to import your DXF to the laser cutter software. Full details are not repeated here as this process is taught properly in the laser cutting training.
4. Use the 'offset curve' tool to reduce the sizes of the pad apertures. The exact size of the offset is left to you, as the size and density of the pads will influence the optimal offset. It's good to aim for approximately 80% of the real pad size, as pressure from the component and application of heat will make the paste expand to cover the full available area.
5. Rotate your stencil by 45 degrees. This is important as it accounts for any slack on either of the laser's principle axes of movement.
6. Set the cut to engrave.
7. Set power to 50, and speed to 500.
8. Ensure the bed is flipped to the flat side.
9. Place your Mylar sheet on the cutter's bed at 45 degrees, to match the cut.
10. Follow the usual procedure with the laser cutter (focusing the laser, configuring the extractor, etc), and you're good to go!