UFE Main PCB First Try

UFE PCB Toner Transfer First Try

 

Last night, I did a test to see how the PnP Blue toner transfer for the top layer of the UFE main PCB would work. As you can see from the photo, the results are quite promising. Except for the upper left corner area where I had to do a few touchups, the results are almost perfect. I wanted to go ahead with the manufacturing at this stage, but then I realised that I forgot to set the proper hole sizes and octagonal shape for the pads. So,  hopefully during the weekend I’ll do another transfer and get it perfect this time 🙂 .

LCD Daughterboard

LCD Daughterboard

 

I’ve completed the layout for the LCD daughterboard. Apart from the usual signal and power connections, I’ve implemented a simple, two-transistor 60ma constant current source for the backlight that is powered from the 5V available from the main board on the header. It may not be absolutely necessary though, since the LCD datasheet specifies a forward voltage drop of 3.2 to 3.4V for 60ma of current. The backlight would probably work fine when directly powered from the 3.3V rail.

Now, all that remains is to do some thorough final checking on both boards and hopefully move on to the manufacturing stage.

UFE Revision A PCB

UFE PCB Revision A Top View

 

For a while now, I’ve been working on UFE, which is the successor to the TFE. It will be considerably more advanced than TFE and (inevitably) more complex and more expensive to manufacture. Some of the planned features are: Full-Speed USB embedded host (enabling use of flash drives as storage instead of SD-MMC card), touch-panel QVGA color LCD user interface (see previous post), read and write support for multiple drives on the same floppy bus, on-board image MFM conversion (using on-board SDRAM).

The picture above shows my first attempt at designing a PCB for the UFE. The main components are a PIC32 microcontroller and a 128Mbit SDRAM chip. The SDRAM is connected directly to the PIC32. The PIC32 has no on-board SDRAM controller, so I plan to emulate it using some software tricks. The same goes for handling of the floppy bus with the SEL lines. If this approach fails, plan B will be to add a flash CPLD to the board to offload some of these tasks to. In this case, I’ll have to redo the design of course,  but this first revision will still give me ample opportunity to experiment with the main system components.

The header on the right is for the LCD module daughterboard. The daughterboard will contain the actual socket for the LCD as well as some additional circuitry such as backlight LED drivers. Decoupling these functions from the main board allows me to maintain a degree of independence from the actual LCD module used, and remove some of the associated complexity from the mainboard.

To be honest, I am not entirely sure that I’ll be able to manufacture this board at home. The reason is mainly the 8/6 rules (8 mil trace width, 6 mil clearance) I had to use in the design due to the PIC32, and the high density of the top layer. My previous experiences tell me that 8 mil traces should not be a problem, but I am not sure about the 6 mil clearance.

2.4″ TFT LCM

2.4″ TFT LCM Back View

2.4″ TFT LCM Front View

 

I got a brand new 2.4″ TFT LCD module off ebay from China. It has 320×240 resolution and 65k colors, a built-in display controller with on-chip VRAM and a resistive touch panel. I plan to interface it with a PIC32 microcontroller for a new project I am working on.

The PCB “Via Press”

The PCB “Via Press” in action

Closeup of vias made with the Via Press

 

When designing double-sided PCBs, it’s crucial to be able to place vias underneath SMD ICs. Otherwise, the boards become too difficult (not to mention ugly) to route. This presents us with a problem when making those boards at home, however. A via must protrude from the board only very little, otherwise it cannot be placed under an IC (So, placing copper wire in the via hole and soldering both ends is out). Since it’s very difficult (almost impossible for most people, including myself) to reproduce the through hole plating process at home, I’ve been investigating practical ways of making vias that can solve this problem.

The best solution I’ve come up with so far  is using “mechanical vias”. In a nutshell, I place a small piece of copper wire in the via hole. It needs to be a tight fit so that it won’t slip out of the hole. I trim the ends of the wire so that very little (perhaps less than a millimeter) of it sticks out of the hole in both sides. Then, I place the board in something like a vice (what I referred to as a “via press” in the post title) and applying pressure to the board crushes both ends of the copper via and produces what seems like a reliable connection between the two sides.

I built a device out of aluminium (see photo) that can be used as a “via press”. I place the board in this press, sandwiched between two stainless steel plates (aluminium itself is too soft to crush the copper wire). The jaws of the device are more than 17cms wide so it can accomodate reasonably large boards. As you can see from the photo(s), the workmanship is far from perfect, but it works and I guess it’s alright given my (lack of) tools and prior experience when it comes to metalworking. Lots of vias can be made in just one pass using this device. So far, as it can be seen from the photos, this method seems to be quite successful :).