Electronics Production: Milling The Board 

This week, I milled my PCB board and stuffed it (soldered the resistors, capacitors, etc. onto the board). The board on the CBA MIT website is hello.D11C.serial.5V.1.1 (located on this website: http://academy.cba.mit.edu/classes/embedded_programming/index.html) 

The milling process was much harder than I thought - I messed up so many times before I got it to come out right! So, here are my tips and tricks to future students for how to get it right:

• If you see that not all of the copper has been removed (e.g I saw this when I used offset = 4 when milling out the interior traces of the inner copper design), use offset 0, which will remove all the copper from the board so it looks nicer. Offset 4 will still make the board functional, but offset 0 is ideal for a cleaner, nicer looking board. 
• If you see jagged edges like the middle image to the left, that means your mill is probably broken/damaged! Ask the TA's to get a new one and that will fix the issue. This happened to me! 
• If your file appears to look fine but looks oddly inverted and/or the traces are wavy/curvy, this may mean that you customized the file incorrectly. I used InkScape and it turned out like the board on the very right of the photo here. Instead, use GIMP to modify the image or otherwise just start from scratch in Eagle to design a new board.

Stuffing The Board (Soldering On The Components):

For the soldering process, I didn't have too much trouble since I had soldered before. However, I did run into a few difficulties. 

 

Above is an image of the different components that I stuffed onto the board (LEFT), and an image of my final stuffed board (RIGHT). 

Here were some of my mess-ups (including using the wrong solder iron tip which was too big and clumsy): 

Unfortunately, while trying to take a video of myself soldering, I accidentally bumped my fingeragainstthe solderiron tip while it was hot. Since 65 degrees Celcius was required to properly melt the solder, with 1 second of exposure at that temperature I got a 1st or 2nd degree burn. Tip to future students: even though the example videos show usage of the solder iron without gloves, you may want to consider getting heat-protective gloves (there are apparently some that are quite recommended for workers to wear while soldering!) 

Group Project: 

We milled a template of differently sized traces onto a board, to see what size trace and what size gaps wold be best for design parameters. 

From this photo below, we can see that the very tiny traces don't end up particularly large enough to solder onto. The optimal trace width is about 0.3 mm, and the optimal distance between the tracesis about 0.4mm. On the image below, 0.0118 inch is optimal for the traces and 0.157 inch is optimal for spacing between the traces. 

Lastly, I tested my PCB board with a multimeter. I connected the black and red connectors to different GND (ground) locations on the board and if it beeped, there was a connection. 

Learnings & More Advanced Applications

• I messed up this week once again at compressing files. The error I made was that I put spaces in the image filenames when using the img_format_updated.py script. Nothing happened. When I swtiched the filename to underscores instead, the script worked just fine! Yay! 
• The smaller (thinner) solder iron tip works WAY better for soldering the PCB boards than the super large solder iron tip! I figured this out the hard way; I was using the larger tip but it was so hard to get into the tiny spaces. When I used the smaller tip, I found that it was so much easier to get into the spaces and use the solder iron tip to melt places where the solder had connected between two different sections in an unwanted way (causing the solder to flow away from the gap and towards the two metal copper sides as needed). 
• I messed up many times when  trying to add solder blobs to the copper sections on the board before "stuffing" (soldering the components onto the board). After a few tries (and with suggestion by another student in the shop), I found that adding just one blob to one spot on the board for each component, this worked much better because I could secure the component on with just one blob in one spot, and then it was much easier to press the solder iron against the board and against the component for the other soldering parts of it, and place the solder wire against the solder iron to allow the solder to flow in the joint area. 
• For more advanced applications, I could have used GIMP to modify the file or Eagle to make my own file for milling out the PCB board. However, I was unable to do so since it took so long to troubleshoot getting a decent board milled in the first place! 

Additional Notes / Modela Instructions

https://www.mokotechnology.com/pcb-milling-board/

How To Mill The Traces: 

1) Use hex to insert & tighten a 1/64" end mill into the equipment. 

2) Open terminal, and click on mods on the desktop. Select programs > Open Server Program > Roland > Mill >  SRM-20 > PCB png (SRM-20 PCB png)

3) Load your file into mods (Select PNG File button), make sure dimensions are right (see photo). Change DPI if needed. (e.g. to 2000 DPI in mods interface for the Roland mill, if I exported PNG with 1000 DPI from Eagle when making my own PCB design) 

4) Click on Mill Traces under Set PCB Defaults section within mods 

5) Origin: Under Roland SRM-20 Milling Machine section in mods, set values and click Move to Origin to move the mill to a specific spot in the lower left corner of your part

The lower left of your image in mods is the origin. In the equipment, y axis is into the machine and x axis is along the width of the machine. 

6) Use Hex Key to looser and bring down the 1/64" end mill to touch the surface 

7) Record your origin values on a piece of paper 

8) Set offset numer (offset = 0, where 0 = fill) 

9) Click Calculate 

10) Click on the "Send File to Device" button 

How To Mill The Outline: 

Same as above, except: 

• Use 1/32" end mill 
• Choose "Mill Outline" instead of "Mill Traces" for the Set PCB Defaults section in mods 

What is Offset? 

offset 1 = less material taken off

offset 4 = takes more off

offset fill = takes it all off. 

Which end mills should I use for milling the outline versus the interior traces?  

Do outline / cutout of the edge of the board with 1/32" end mill

Do the exterior (main design) with 1/64" end mill 

Add PNG

Set origin

Unscrew end mill with hex and make it go down. 

Move to origin

Mill outline

Calculate

Send to device 

Click "Mill Outline" instead of "Mill Traces" when doing the outline, & use 1/32" mill for the outline/outer cutout 

If it blinks, turn it off and on again

To scale, load PNG and view the bottom dimensions in mods, e.g. 0.906" x 1.520" for this board template they gave us 

ERRORI I MADE: 

I did not tighten the hex tight enough! (mill was loose)

Offset 4 

Set offset number to (0=fill) will make sure all the copper comes off :) 

Oopsie - I loaded my file and it didn't work. Needed to invert it at least possibly ?? 

Oopsie 2 - Used a broken mill fort he outline and it was super jagged and thick cutout line 

It gave jagged outline 

we used fill = 0 this time and then tried again, and with a new fresh mill from the TA.