Summary of UV Exposure Unit & Etching
### Summary This article details a DIY method for manufacturing SMD circuit boards using photoresist and UV exposure, overcoming the limitations of toner transfer. The author constructs a low-cost UV exposure unit by repurposing an old scanner housing and a face tanner containing four UV tubes. The process involves printing layouts on transparencies, exposing photoresist boards for approximately 90 seconds, developing them in sodium hydroxide, etching with a hydrochloric acid and hydrogen peroxide mixture, and finally cleaning residual resist with isopropanol.
Parts used in the DIY UV Exposure Unit:
- Broken scanner (older models preferred)
- Face tanner (containing 4 UV tubes, 2 trafos, and 4 starters)
- Hot glue
- Inkjet-printed transparency layouts
- Photoresist boards
- Sodium hydroxide solution (10%)
- Hydrochloric acid
- Hydrogen peroxide
- Isopropanol
Manufacturing circuitboards containing SMD-parts with toner-transfer is nearly impossible. The thin traces almost never get transferred completely to the copper. One solution to this problem is to use photoresist boards and expose them with UV light.
The layout is printed onto a transparency, which is then used as a mask for the photoresist. I got the best results with inkjet-printed layouts. They seem to be more opaque than laserprinted ones.The cheapest UV exposure unit from Reichelt costs 200€. That’s far too much for what is essentially some UV-light source and a glassplate.
A much cheaper way to get one, is to build it yourself.
So i got a broken scanner (the older, the better (bigger housing)) and a face tanner (not broken, but old) from eBay. The tanner contained 4 UV tubes, 2 trafos and 4 starters. All this was glued into the gutted scanner with hot glue. About 90 seconds per layout seem to sufice for good exposure. The boards are then developed in a 10% sodiumhydroxide solution and etched in a mixture of hydrochloric acid and hydrogen peroxide. (More information on this etching solution in this great tutorial). The remaining photoresist can be removed with isopropanol. For more Detail: UV Exposure Unit & Etching
- Why is toner-transfer difficult for SMD parts?
The thin traces almost never get transferred completely to the copper. - Which printing method yields better opacity for masks?
Inkjet-printed layouts seem to be more opaque than laser-printed ones. - What components were salvaged from the face tanner?
The tanner contained 4 UV tubes, 2 trafos, and 4 starters. - How long does the exposure process take?
About 90 seconds per layout seems to suffice for good exposure. - What chemical is used to develop the boards?
The boards are developed in a 10% sodium hydroxide solution. - What mixture is used for etching the copper?
A mixture of hydrochloric acid and hydrogen peroxide is used for etching. - How can remaining photoresist be removed?
The remaining photoresist can be removed with isopropanol.
