Bus Pirate SOB case review

Standard PCB templates and project cases

Bus Pirate SOB case review

Postby dsm » Fri Dec 30, 2011 4:50 pm

background comments

In order to gain some experience with the tool chain I want to use to fabricate case prototypes for another project,
I've been designing some custom cases for the Open Bench Logic Sniffer and the Bus Pirate. The relevant Dangerous Prototypes forum postings for these cases can be found at yaobls (link) and yabp (link).

The fabrication approaches I investigated included the following:
  • Modified off-the-shelf case (milled or laser cut openings).
  • Custom milled case.
  • Laser cut acrylic (layer stacking, cement bonding, tab and slot, cross and slot, etc.).
  • FDM (fused deposition modeling) 3D printer (online service, personal commercial, personal open source).
  • SLS (selective laser sintering) 3D printer (online service).
  • Custom injection molded case.
For various reasons, I've been prototyping my designs using SLS (selective laser sintering) in nylon through the Shapeways 3D printing service. The resulting parts are amazing strong (not brittle), slightly flexible (see yabp push-button plunger detail), and comparatively cheap (compared to most subtractive machining processes). The main downside to using Shapeways is that prototypes typically take a couple weeks to arrive.

For my case prototypes, I want a packaging technology that allows designs that are
  • Relatively thin (vertically).
  • Relatively detailed.
  • Relatively robust.
  • Relatively cheap.
  • Allows path to injection molding.
  • Clear or smoky plastic to allow box contents to be viewed (want rather need).
For example, the most recent versions of the yaobls and yabp cases have been 12.0 mm thick.
    1.5 mm (top-wall-thickness) +
    5.0 mm (space-above-board) +
    1.7 mm (board-thickness-plus-soldermask-and-silkscreen(s)) +
    1.8 mm (space-below-board) +
    1.5 mm (bottom-wall-thickness) +
    0.5 mm (foot)
    = 12.0 mm
The top-wall-thickness is limited to 0.7 mm by Shapeways.
The space-above-board is limited by the length of the light-pipes I'm using.
The board-thickness-plus-soldermask-and-silkscreen(s) is typically between 1.6 mm and 1.7 mm.
The space-below-board is limited by the solder tail length if you use through-hole connectors.
The bottom-wall-thickness is limited to 0.7 mm by Shapeways.
The foot is 0.5 mm to protect the case bottom and allow screws with thicker heads.



case comments

I like the notion of determining a small number standard board sizes to reduce the number of enclosure sizes.
Seeed Studio's Fusion PCB Service board sizes are as good a starting point as any. I believe people are particularly interested in small rectangular boards for devices like the Bus Pirate and the OBLS that are likely to be laid out using tools like the free versions of CadSoft Eagle (link). Any boards larger than 80 mm x 100 mm are therefore out of context for many people.

For example, 50 mm x 50 mm Fusion PCB Service board ~ suggests 30.9 mm x 50.0 mm ~ golden ratio rectangular board.
(Use 31 mm x 50 mm so the board major and minor axis center-lines are on a 0.5 mm grid.)
For example, 50 mm x 100 mm Fusion PCB Service board ~ suggests 50.0 mm x 80.9 mm ~ golden ratio rectangular board.
(Use 50 mm x 81 mm so the board major and minor axis center-lines are on a 0.5 mm grid.)
For example, 100 mm x 100 mm Fusion PCB Service board ~ suggests 61.8 mm x 100 mm ~ golden ratio rectangular board.
(Use 62 mm x 100 mm so the board major and minor axis center-lines are on a 0.5 mm grid.)

Note that the old Bus Pirate boards (at 65 mm x 40 mm) were close to being a golden ratio rectangular boards.
Note that the new Bus Pirate v3.5e and v4.0c boards (at 37 mm x 60 mm) are close to being a golden ratio rectangular boards.

Lower case phi aka golden ratio is (1+sqrt(5))/2. See Wikipedia article (link) for more information.

Thanks for your time.

dsm
Last edited by dsm on Wed Jan 04, 2012 5:11 pm, edited 3 times in total.
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Re: Standard, beautiful PCB cases

Postby ian » Fri Dec 30, 2011 5:09 pm

Thanks dsm :)
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Re: Standard, beautiful PCB cases

Postby dsm » Sat Dec 31, 2011 12:42 am

question

Where did the new 60 mm x 37 mm Bus Pirate board dimension come from?
Did you have a particular enclosure in mind?



correction
dsm wrote:In general, you want B > A.

Hmmm... I think I should have said A > B. Sorry about that.

As the screw head diameter increases, the distance from center-of-mounting-hole to edge-of-board needs to increase.
Note that the bottom case mounting hole boss is larger (since it is determined by the screw head diameter), but does not interfere with the IO connector shell (which is on the top of the board). The top case mounting hole boss is smaller (since it is determined by the screw thread diameter) and is unlikely to interfere with the IO connector shell.

clamshell-yabp-case-foot-screw-detail-v12.jpg
clamshell yabp case foot screw detail



general board layout suggestions

See Bus Pirate board layout suggestions on 19Sep11 yabp (link) forum posting.

In addition...
Anything that interacts with external geometry - mounting holes, connectors, LEDs, push-buttons - would benefit from being locked down to a coarser grid - say 0.5 mm or even 1.0 mm. As a general comment, if you keep changing the position of the above components it becomes almost impossible to economically design an enclosure (as well as very frustrating).

Since two-sided PCB material is comparatively cheap, I suggest using a slightly larger board than the minimum required.
This tactic has several advantages.
  • First, a slightly larger board allows more freedom to position components that interact with the external geometry - mounting holes, connectors, LEDs, push-buttons - as needed.
  • Second, minor local changes to the design don't ripple upward through the design hierarchy.
  • Third, multiple designs are more likely to be able to use the same board sizes and enclosures.
When describing a board layout, just saying BusPirate-v3.5e-SSOP.brd or BusPirate-v4c.brd seems to be insufficient. Given how components move around before a layout is released, how about appending the release number like BusPirate-v3.5e-SSOP.brd [r1606] or BusPirate-v4c.brd [r1597]? The letter suffix seems to be related to the schematic, while the revision number seems to be related to a specific layout of that schematic. Is that what you actually do?



Bus Pirate layout specific comments

Position the LEDs on the board to reflect the way you want them to look on the exterior of the case.
For example, rather than shifting the LEDs off to right like BusPirate-v4c.brd [r1606], how about positioning the LEDs so that they are symmetrically positioned (e.g. 12.0, 24.0, 36.0, 48.0 mm in "X" dimension and 34.5 mm in "Y" dimension)?

Position the push-buttons on the board to reflect the way you want them to look on the exterior of the case.
How about 4.9 mm in the "Y" dimension?

Position the ICSP connector on the board to reflect the way you want it to look on the exterior of the case.
How about centered between the two push-buttons and with the same "Y" dimension as the center of the push-buttons?

Position the IO connector on the board to reflect the way you want it to look on the exterior of the case.
Move the IO connector (center) to (52.0, 18.5) to free up some space.

Allow plenty of room for mounting holes. 3.8 mm from center-of-mounting-hole to edge-of-board would be good, but 5.0 mm would be even better.

Is there any good reason to for the mounting holes to be PTH? The plating changes the diameter of the hole, gets ground up by screw threads, and has no obvious purpose unless the case is metal.

Thanks for your time.

dsm
Last edited by dsm on Tue Jan 03, 2012 12:49 pm, edited 2 times in total.
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Re: Standard, beautiful PCB cases

Postby ian » Sat Dec 31, 2011 2:46 am

Thanks so much for taking the time to give us your feedback.

5cm Max*5cm Max
5cm Max*10cm Max ( +$15.00 )
5cm Max*15cm Max ( +$25.00 )
5cm Max*20cm Max ( +$35.00 )
10cm Max*10cm Max ( +$15.00 )
10cm Max*15cm Max ( +$60.00 )
10cm Max*20cm Max ( +$70.00 )
15cm Max*15cm Max ( +$95.00 )
15cm Max*20cm Max ( +$120.00 )
20cm Max*20cm Max ( +$120.00 )

30x50
37x60
43x70
49x80
56x90
62x100


These were my original case design sizes. Approximately, but not exactly, golden ratio.

Mounting holes do not need to be plated, we can make then simple drills.

The version number is based on the board we send. When we commit to a PCB then we increment the minor version. There's not a lot more logic to it, the SVN revision number would probably be helpful.
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Re: Standard, beautiful PCB cases

Postby dsm » Mon Jan 02, 2012 5:44 am

BusPirate-v3.5e-SSOP.brd [r1619] comments
BusPirate-v4c.brd [r1619] comments

I looked at the BusPirate-v3.5e-SSOP.brd [r1619] board layout and have a couple comments.
I looked at the BusPirate-v4c.brd [r1619] board layout and have a couple comments.

  • Both board outlines are 60.0 mm x 37.0 mm which is approximately golden ratio to the nearest 1.0 mm.
    This allows the board center-line to be on a 0.5 mm grid. Nice.
  • Mounting holes are symmetrically 3.41 mm in from the corners. Same positions on both boards.
    3.41 mm seems like a pretty odd dimension (i.e. doesn't seem to correspond anything obvious in metric or english systems). Would it make more sense for this dimension to be 3.5 mm or 4.0 mm or 4.5 mm or 5.0 mm?
    Maybe not - see below.
    Then the mounting holes would be on a 0.5 mm grid like everything else on the board.
  • Connectors are on a 0.5 mm grid. Same positions on both boards. Nice.
    You could shift the IO connector slightly closer to the right edge to gain some additional space.
  • Push-buttons are on a 0.5 mm grid. Nice.
    The Normal push-button is 19.5 mm from the left side. The Reset push-button is 16.5 mm from the right side.
    You could shift the Normal push-button 3.0 mm left for more symmetry.
  • LEDs are on a 1.0 mm grid that looks symmetric on the exterior enclosure. Same positions on both boards. Nice.
Other than the mounting hole stuff, the only other concern I have is the spacing between the ICSP connector and the Reset push-button. If you end up using a cantilever plunger scheme to reset the Bus Pirate when the enclosure is closed, the cantilever may not have enough margin compared to the ICSP opening envelope. If the ICSP connector were more centered between the two push-buttons, this issue mostly goes away.

IMG_0753-edit.jpg
clamshell-yabp-case-dsm-v3 (assembled ~ top view)

Bus-Pirate-v3.5e-board-geometry-v0151-xlsx.pdf
Bus-Pirate-v3.5e-board-geometry-v015.1 (minor edit)
(56.84 KiB) Downloaded 350 times

Bus-Pirate-v4c-board-geometry-v0151-xlsx.pdf
Bus-Pirate-v4c-board-geometry-v015.1 (minor edit)
(57.3 KiB) Downloaded 317 times

My $0.02 worth.

Thanks for your time.

dsm
Last edited by dsm on Wed Jan 04, 2012 5:58 pm, edited 3 times in total.
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Re: Standard, beautiful PCB cases

Postby ian » Mon Jan 02, 2012 10:23 am

Thank you so much DSM,

We will prepare an update with:
Holes - not plated, up to 5mm center of hole to edge of board, on the 0.5mm grid
Buttons - The Normal push-button is 19.5 mm from the left side. The Reset push-button is 16.5 mm from the right side.
You could shift the Normal push-button 3.0 mm left for more symmetry.
ICSP header - ICSP connector were more centered between the two push-buttons, this issue mostly goes away.
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Re: Standard, beautiful PCB cases

Postby dsm » Tue Jan 03, 2012 5:12 am

acrylic comments

Acrylic sheets are available from manufacturers in both english and metric thicknesses. For example, 1/8" and 3.0 mm or 1/16" and 1.5 mm. Since most people are only slightly interested in the thickness, vendors are often quite sloppy about specifying the actual thickness of their products. Make sure you check your acrylic thickness carefully. If you're building standard, beautiful acrylic cases with slots and tabs, the actual thickness of the acrylic is obviously important.

An english tab into a metric slot may be too tight.
An metric tab into an english slot will may be too loose.

The kerf of your laser cuts may vary depending on how your laser cutter is adjusted. Also the laser cut itself will be slightly wider at the top and narrower at the bottom. This effect is relatively greater in thicker acrylic than in thinner acrylic.

3.0 mm acrylic is probably overkill from a strength viewpoint. Using 1.5 mm acrylic is probably strong enough and will reduce your materials cost by roughly 40%. The only design change is that you might want to consider is using multiple short tabs and slots instead of one big tab and slot.

Another advantage of using thinner acrylic is that it may bend enough to allow the cantilever plunger scheme to activate a push-button when the enclosure is closed.



external case frame

Another type of enclosure design where the acrylic thickness matters is the external case frame. The following example uses 12x extruded edge segments and 8x laser cut acrylic surface windows. The acrylic surface windows are held in grooves in the edge segments. The case is assembled using cyanoacrylate adhesive, mechanical fasteners with barbs, or ultrasonic welding. An ordinary spreadsheet is used to generate the edge segment/surface window cut instructions for any size enclosure.

case-frame-60.2x37.2x8.5-assembled-acrylic.jpg
BusPirate-case-frame 60.2 x 37.2 x 8.5 (assembled)

case-frame-60.2x37.2x8.5-assembled-acrylic-three-quarter.jpg
BusPirate-case-frame 60.2 x 37.2 x 8.5 (assembled ~ three-quarter cutaway)

Thanks for your time.

dsm
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Re: Standard, beautiful PCB cases

Postby Sjaak » Tue Jan 03, 2012 5:38 am

ian wrote:Thank you so much DSM,

We will prepare an update with:
Holes - not plated, up to 5mm center of hole to edge of board, on the 0.5mm grid
Buttons - The Normal push-button is 19.5 mm from the left side. The Reset push-button is 16.5 mm from the right side.
You could shift the Normal push-button 3.0 mm left for more symmetry.
ICSP header - ICSP connector were more centered between the two push-buttons, this issue mostly goes away.


Is there need for the ICSP connector to be accessable? I think the case looks nicer without one (the same for the Vusb jumper).
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Re: Standard, beautiful PCB cases

Postby ian » Tue Jan 03, 2012 6:40 am

Thank you for your notes on Acrylic. One of the things Tayken had mentioned is getting a sample cut by Seeed so we can see the tolerance of the cut.

I like the enclosed case. Is there a way to combine it with mounting holes? Through the acrylic on one side maybe? The nut could be capped with a rubber foot. Just a thought.

@Sjaak - on our case design I agree, I am not concerned about ICSP and Vusb jumper accessibility. If other people like DSM want to make a case that exposes the headers I figure we should give them the opportunity though.
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Re: Standard, beautiful PCB cases

Postby fcobcn » Tue Jan 03, 2012 9:01 am

I really want to jump into this as soon as I finish some work.
Question: Is it possible to cut 1mm acrylic with seeed's machine?
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Re: Standard, beautiful PCB cases

Postby Jer » Tue Jan 03, 2012 9:04 am

Hi DSM,

Thanks for your notes. Do you think moving the holes 4mm from the corners would be enough? I'm afraid there wouldn't be enough room for the shrouded IO connector (for the Bus Pirate v4) if the holes are moved beyond 4mm from the corners
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Re: Standard, beautiful PCB cases

Postby ian » Tue Jan 03, 2012 9:39 am

Is it possible to cut 1mm acrylic with seed's machine?


I'm sorry, I don't know the exact specs. We will be prototyping with Polulu though, and I'm pretty sure they have 1mm acrylic.

http://www.pololu.com/docs/0J24/3#acrylic
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Re: Standard, beautiful PCB cases

Postby Jer » Tue Jan 03, 2012 10:14 am

Looking at the clamshell foot screw diagram: Should 3mm and 4.2mm from the center of the holes be defined as component keepout areas for the top and bottom respectively?
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Re: Standard, beautiful PCB cases

Postby tayken » Tue Jan 03, 2012 10:58 am

dsm wrote:3.0 mm acrylic is probably overkill from a strength viewpoint. Using 1.5 mm acrylic is probably strong enough and will reduce your materials cost by roughly 40%. The only design change is that you might want to consider is using multiple short tabs and slots instead of one big tab and slot.

Thanks for the tips dsm. One reason why 3mm acrylic is my material of choice is as standard pin headers are around 12 cm high, I can make a case by stacking 4 cut sheets +1 for the bottom which makes a total of 5 pieces. + it is widely available & cheap here.

I had the same idea with multiple short tabs, that design was a preliminary one, I'm planning to make longest tab length about 2-3 cm max.

dsm wrote:Another type of enclosure design where the acrylic thickness matters is the external case frame. The following example uses 12x extruded edge segments and 8x laser cut acrylic surface windows. The acrylic surface windows are held in grooves in the edge segments. The case is assembled using cyanoacrylate adhesive, mechanical fasteners with barbs, or ultrasonic welding. An ordinary spreadsheet is used to generate the edge segment/surface window cut instructions for any size enclosure.

Those pics look really familiar. Are you also using Solidworks for modelling?

fcobcn wrote:Question: Is it possible to cut 1mm acrylic with seeed's machine?

That depends one material availability. For beam width test purposes, we can even do with some wood or other material IMO.
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Re: Standard, beautiful PCB cases

Postby dsm » Tue Jan 03, 2012 11:12 am

connector access

Sjaak wrote:Is there need for the ICSP connector to be accessable? I think the case looks nicer without one (the same for the Vusb jumper).

Since the acrylic case parts are laser cut, I suppose you could offer case parts with and without holes for connectors.

Since I am generally trying to make thin enclosures (i.e. top-wall-thickness + space-above-board < height-of-connector),
the various connectors in my designs (if loaded) typically stick out of the top of the enclosure. Since I am also concerned about shorts, I often use female headers (which is the opposite of the usual practice). When I want to use a connector, I just insert a a strip of male pins (18 mil round pins instead of 25 mil square pins) into the exposed female header. This approach requires accurate relative positioning of the connectors on the board and the holes in the case.

Another scheme is to not attach the ICSP connector to the board at all, but instead insert a bare male header into the ICSP footprint (or use pogo pins) as needed. Apply sideways force to the male header or use a staggered array footprint or offset the pins in the male header to maintain electrical contact. Sparkfun once ran an article about the optimum offset of pins in a staggered array footprint for this application. You could access the bare ICSP footprint from the bottom of the case, which would then not visible from the top of the case.

If your case is thick enough (i.e. top-wall-thickness + space-above-board > height-of-connector), opening the case to access the various connectors for development starts to affect your choice of case attachment schemes. The proposed cases seem to require literally disassembling the case to access internal connectors.

I suspect that the most flexible solutions include the following:
  • Eliminating the need for external connectors.
  • Make connectors permanently accessible on the outside of the enclosure.
  • Make it easy just to just pop apart a case apart that is held together with magnets (or some other mechanical scheme).
My $0.02 worth.

Thanks for your time.

dsm
Last edited by dsm on Thu Jan 05, 2012 5:48 pm, edited 4 times in total.
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