SPI Backpack PCB for Liquid Crystal Displays: Part 1 Design & Print
In my previous posts here, I constructed Controller Area Network shields using single sided PC boards. They work well, but now I want to combine the MCP2515 CAN driver circuit with a 74HC595 serial-parallel converter so that an LCD display and CAN network can share the SPI buss. This is more complex than the CAN shield alone so a double sided PC board is in order. The two circuits I’m referencing are both available in Eagle board files. After about a week of learning Eagle and dozens of Auto route passes, the following design emerged.
I’m using a laser printer toner transfer method to lay out PC boards. This board is the same physical size as the common 16×2 LCDs, 3 1/8″ x 1 3/8″. Ironing on the toner spreads it a little so I need a design that has plenty of clearance. Since there are no plated through holes, I need to minimize vias, and force wiring to the bottom layer if it is not possible to solder the component on both top and bottom. It amazes me how nudging a component a couple of millimeters can drastically change Eagle’s routing.
There are many web pages describing how to etch PC boards using laser printer toner as resist material. There is an active forum on Yahoo with lots of information. Results reported vary a lot depending on the printer and paper used. All toner formulas are not equal.
Laser toner is ground up plastic with black material mixed in. Letters are formed by electrostatically attracting toner particles to the paper, then melting it in over a heated fuser roller. It is possible to re-melt the toner and transfer some of it to the copper surface of a blank PC board. The paper that works best is high quality photo grade stock, but thin. This type of paper has a high clay content to make the surface glossy, and ink that hits the clay drys immediately and won’t smear. Slick magazines like these qualities, so many people use magazine paper to print their PC board patterns.
My laser printer is an old Lexmark 4039 10+. It has an extra dark setting that makes toner transfer easier, and has a rear single sheet paper feeder and front exit slot. This gives a very straight paper path so the printed copy doesn’t curl up. Here is a photo of the two prints needed to make my LCD backpack. The paper is torn from an old Digikey microcontroller catalog. Note: always cut the torn edge off with scissors before running through the printer else it will catch inside and jam. Don’t Ask Me How I Know This.
It’s difficult to visualize how the top and bottom of the design (top photo) is going to work when applied to the PC board (Don’t Ask Me How I Know This either). Eagle has a “mirror” button in the print requester. You use this when printing the top layer, but not when printing the bottom layer. If you did it right the two prints will match up when placed face to face. Why mirror the top and not the bottom? Remember that both images are mirrored when they are transfered to the PC board – so the top gets mirrored twice resulting in a non mirrored resist, and the bottom gets mirrored once resulting in a reversed image. Eagle knows this. Any text Eagle puts on the bottom layer is automatically backwards on your screen but prints correctly on the PC board.