5. Electronics production

Process Characterization: PCB production

Mill Test Traces

The fab academy test trace file above was use to create cam to test our X-Carve 500mm size milling machine. I purchased the smallest 500mm model of the X-Carve mill because it would be more rigid and so it would fit on a normal work bench top.
I had been told that it was impossible to micro-machine or mill a circuit board on a lower cost (sub $1000) milling machine like an x-carve. I suspected that this was not true, so I proceeded in the following way. It seemed to me the mass of the spindle, small cutting diameter, and rigidity of the machine was capable. I did know that vibration and concentricity of the tooling in the spindle may be important.
I wanted to make sure my x-carve spindle was ready for micro-machining. The first few times I tried to machine my circuit board on my x-carve it broke the 1/64” diameter end mill almost immediately. The bit also seemed to be vibrating a lot despite the fact that the spindle itself was runny vibration free and quietly. Being trained as a machinist years ago, I knew I should check the spindle and tooling run-out of the 400watt 48vdc quiet cut spindle we have installed on our x-carve mill. Below you can see photos where I am using a Mitutoyo 513-406-10E Dial Test Indicator to check the runout. This specific Dial Test Indicator was suggested by Ron Reed at Precise Bits a micro machining supplier I called in the process of researching micro-machining.

After rotating the tooling, collet and spindle the Total Indicated Run-out (TIR) measures a max of .0015” which is OK for a 1/64” diameter bit, but could be improved. I am waiting for some new collets from Precise Bits and we will see if they can improve TIR.
The above test trace file was cut with the following parameters:

Feedrate	Speed	Plunge Rate	Cut Depth	Tool	TIR
10 ipm	12000rpm	4 in/min	.004 in	1/64” 2FL CU 129974	.0015 in

The test traces came out quite good for the purposes of the Fab Academy and matched the typical output of a Modela mill. The traces stayed in place into the 4th segment from the left, this means all typical trace widths will be easily machined on our x-carve. If I have time I will further optimize the x-carve with a focus on further reducing TIR.

The section above satisfies the group assignment assessment requirements of: 1. Characterize the design rules for your PCB production process: document feeds, speeds, plunge rate, depth of cut (traces and outline) and tooling. 2. Document your work individually

Making: FabTinyISP

After being unable to program a ARM based JTAG programmer I made, I checked in with the Saturday Zoom help group. Pablo and Adrián from Fab Lab Leon both suggested that I make a FabTinyISP first and then move on to other boards. I took their advice and followed these instructions to make a FabTinyISP

I will return to my ARM JTAG Programmer project at a later date and my work and documentation will be updated here: JTAG Programmer

Milling

I used the same steps to mill the board as documented for my JTAG Programmer.

Establishing Feed Rate

Calculations for a PreciseBits EM3E8-0150-15V Tapered Stub End Mill were completed using:

https://www.precisebits.com/products/carbidebits/tapered_stub_125.asp under the Feeds / Speeds Tab

For a 0.0150” (0.38mm) dia - multiply your peak spindle RPM by 0.00381 (dry) or 0.00492 (with ME Lube ) with 20 IPM z-axis plunge rate

Mods suggest 4 mm/s feed, which is 9.5 ips. I’m guessing that this is a recommended starting speed?

The calculated feed rate for my 0.0150” (0.38mm) dia Tapered Stub End Mill is:

Multiply your peak spindle RPM by 0.00381 (dry) or 0.00492 (with ME Lube ) with 20 IPM z-axis plunge rate.

Dry: 12000 * 0.00381 = 46 IPM (19.5 mm/s) Lubed: 12000 * 0.00492 = 60 IPM

I will try dry machining first: 46 IPM (19.5 mm/s)

19.5/4 mm/s= 4.875 IPM

So a feed rate of 46 IPM (19.5 mm/s) is ~4.875 times the feed rate of 4mm/s I was using originally on mods. TIR checks and using the feed calculations to obtain the correct chip load for my 0.0150” (0.38mm) dia 3 flute Tapered Stub End Mill Part Number EM3E8-0150-15V really paid off. Circuit boards will be much faster to make on my x-carve 500x500 mill going forward. The x-carve costs around $1,900 with all the parts needed to get started. The PreciseBits EM3E8-0150-15V mill I used costs $16.15 It is very durable due to its 15deg taper geometry. I thin this new setup will be better for beginners and will prevent bit breakage. It is also faster than our previous process.

Machining at 46 IPM (19.5 mm/s)

Stuffing

Now time to pick out the parts needed to populate the board!

FabTinyISP BOM:

Item	Qty
ATtiny 45	1
1kΩ resistors	2
499Ω resistors	2
49Ω resistors	2
3.3v zener diodes	2
red LED	1
green LED	1
100nF capacitor	1
2x3 pin header	1

Soldering

I used my plugable USB microscope to document stuffing soldering.

Adding a USB plug

I was having frequent communications issues with the as designed USB inerface. I decided to add an off the shelf surface mount USB A Connector. After this my boards no longer had any communication issues.

Functioning: Circuit Board

I programmed my first FabtinyISP board the Atmel ICE programmer.

I then used my new FabtinyISP to program my other three FabtinyISP boards I had milled and stuffed.

Three of my FabtinyISP boards worked, on did not, i put that board aside for trouble shooting later. On two of my boards I mixed up the 499Ω and 49Ω resistors. This prevented programming of the fuses, but seemed to not impact flashing. After removing and replacing the resistors I re-flashed all the boards and everything worked. I used the steps at FabTinyISP Site to program, set fuses and test my boards. The steps taken were:

1. Use a USB 2.0 Mini Hub with Power Switch from Adafruit
2. Linux Command: make flash
3. Linux Command: make fuses
4. Unplug both programmer and my board
5. Plug in my board
6. Linux Command: lsusb outputs Multiple Vendor USBTiny
7. Plug in both programmer and my board
8. Linux Command: make rstdisbl
9. Remove solder bridge

Hero Shot: Working Board

Here is a photo of my working boards:

Here is the output of the lsusb command, you can see all three of my working FabTinyISP boards listed and highlighted in white:

Case for FabTinyISP

I designed and 3D printed case for my new FabTinyISP’s. I hot glued my boards into the case. Then I hot glued a header reinforcement frame around the isp header. I marked the frame with a black dot indicating pin one.

Onshape CAD for Case

Assessment self check

Have you?	Done
Group assignment	Yes
Individual Assignment	Yes
Documented how you made the board	Yes
_Mill	Yes
_Stuff	Yes
_Solder	Yes
Functional Board	Yes
Problems fixed documented	Yes
Hero Shot	Yes

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