Screen Print and Reflow SMT Boards at Home

These test results show the ovens and their associated thermal reflow profiles. You can see how close they are in capability.

Considerations Regarding Solder Paste and Components.

Solder Paste: There are lots of solder paste manufacturers. Some don't provide printable paste in containers less than 500 gr. For a small number of PCBs this would be expensive. What you want is standard eutectic tin/lead solder. This is typically available in either small jars or in the syringes that are typically used in rework stations. Most vendors providing home prototype stencils will have small jars of solder paste available, like the one in the following photo. Be sure to check on this. Solder paste in small quantities cost around $0.25 to $0.50 per gram.

Paste for rework typically comes in 100 gr. syringe sizes, but also has a lower percent of metal so the paste will flow through the syringe easier. Repair paste may slump on fine pitch components causing solder shorts. On QFPs you may have to clear solder shorts with a fine tip solder iron. Because of this, some manufacturers do not recommend using this type of paste for screen printing applications. But it's your choice.

A Few Cautions:
Solder Paste: Most standard solder paste is shipped in a cold pack. It has a longer shelf life if refrigerated. Before using the solder paste remove it from refrigeration and let it warm to room temperature (DO NOT STORE SOLDER PASTE IN REFRIGERTOR WITH FOOD). If you do not have a safe place to refrigerate the solder paste store it in a cool dry place. Some prototype stencil vendors and solder paste vendors can provide a type of solder paste that does not require refrigeration, so again, be sure to check on this.

Solder paste contains lead. You have to dispose of unused solder paste and rags with solder paste on them as hazardous material. Talk to your solder paste supplier about taking this hazardous material back for recycling. If the solder paste supplier won't take this material back, he can usually tell you how to dispose of it safely.

Components: There are far too many types of SMDs to go into here. If you want it done, there is a device that can do it for you. Ask your component manufacturer or supplier.

All SMD component manufacturer provides the solder process thermal requirements, for the components they produce. Always request this and control the oven profile to conform to this data.

Some SMT components have restricted thermal process requirements. Be sure to verify that all components can be processed using a standard SMT process as described in this procedure before you buy them.

The leads on all SMDs are delicate, some more than others. The finer the pitch, the easier to damage the leads. Handle as little as possible and if applicable, keep in their holders until ready for placement. It's best to use a vacuum wand to place very fine pitch devices.

BUILDING YOUR BOARD

1. Set up Your Prototype Stencil
Important: All work surfaces should be conductive and grounded.

Prototype Stencil: The prototype stencil is a stainless steel foil (5 to 10 mils thick) with openings cut though to allow the solder paste to be deposited on to the PCB lands (Pads). To prevent the PCB from moving you can tape it down or you can make a smaller square and use it as indicated.

• Place the large L-shape board holder on a flat surface and tape it down to prevent it from moving.



• Place the PCB into the board holder.



• Place the small L-shape board holder next to the PCB and tape it down to prevent it from moving.



• Align the prototype stencil over the PCB SMT land pattern (SMT Pads) and tape it to large L-shaped board holder.