Mounting PCBs in diecast aluminum boxes? - Page 1
Forum section: electronics / general

author: jmw
author_rank: Frequent Contributor
timestamp: May 27, 2018, 04:53:56 pm
content: Aluminum projects boxes like this look nice and sturdy for my project:But I'm unsure how to mount a PCBA inside. My google image searching shows a lot of empty boxes like above, hardly any example projects, and none where they've highlighted the assembly. The first thing that comes to mind is to drill and tap the bottom below the mounting holes in my board, and then insert standoffs and fasten the board into those. The walls of the box are typically 2mm thick, so pretty much all standoffs will have threads protrude from the bottom, which I don't want. I'd like the sides basically smooth so it will lay flat and avoid marring other surfaces.Is there a better way to use these boxes? Would love to see some examples if people have them!

author: Bud
author_rank: Super Contributor
timestamp: May 27, 2018, 05:07:04 pm
content: Why not use countersink holes/screws with dual internal treads standoffs, or mount your standoffs backwards, i.e. flat side to the wall with countersink screws and threaded side up to the PCB. Just mount your PCB to the standoffs using nuts.

author: gamalot
author_rank: Super Contributor
timestamp: May 27, 2018, 05:19:10 pm
content: Things will be a little bit easier if you have a 3D printer.

author: jmw
author_rank: Frequent Contributor
timestamp: May 27, 2018, 05:27:08 pm
content: Bud: ok, if I do countersunk holes from the outside and then put in flush flat-head screws, I can fasten them to threaded standoffs like these:https://www.mcmaster.com/#standard-threaded-standoffs/=1d0xqrmThey'll accept screws on the other end, so no need to use nuts. I think this is a pretty good solution!

author: SeanB
author_rank: Super Contributor
timestamp: May 27, 2018, 05:34:30 pm
content: Some work with coarse sandpaper to reveal a nice new surface inside in spots, and then epoxy some standoffs into the case, after degreasing both inside of case and standoff, works well.

author: Ian.M
author_rank: Super Contributor
timestamp: May 27, 2018, 06:07:55 pm
content: It may be preferable to use Bud's suggestion to mount to the lid, not the box bottom, slightly over-countersink the screw holes, fill them with epoxy putty after assembly and skim to get a smooth surface, then hide all the ugly under a stick-on front panel decal.

author: Gyro
author_rank: Super Contributor
timestamp: May 27, 2018, 07:04:40 pm
content: I often use diecast boxes upside down, with the lid as the base. It gives a nicely rounded top without the join being as visible. Diecast boxes are a pleasure to drill, cut and countersink - no burrs.

author: bd139
author_rank: Super Contributor
timestamp: May 27, 2018, 07:16:37 pm
content: I'll get shot for this but I tend to only do single sided boards / dead bug in these and just glue them in with epoxy.

author: Kean
author_rank: Unknown
timestamp: May 28, 2018, 11:16:04 am
content: Quote from: Ian.M on May 27, 2018, 06:07:55 pmThis is basically how I've done it, PCB attached to lid (for ease of access) via countersunk machine screws.Rather than just a decal, I've covered the box with a few coats of enamel spray paint.Photos show my penguin weighbridge load cell amplifier built using this method, and are used in some reasonably harsh conditions.

author: fcb
author_rank: Super Contributor
timestamp: May 28, 2018, 11:26:04 am
content: Did you say "penguin"...?

author: Kean
author_rank: Unknown
timestamp: May 28, 2018, 11:45:32 am
content: Quote from: fcb on May 28, 2018, 11:26:04 amYes, why?Video:The data from one of the weighbridges is also fed in real time to an app you can download to your phone

author: gamalot
author_rank: Super Contributor
timestamp: May 28, 2018, 01:17:26 pm
content: Quote from: Kean on May 28, 2018, 11:16:04 amHi Kean, I'm sorry this may be a little off topic, I guess I could recognize almost all the devices on your board: MCU, EEPROM?, ADC, RS232 transceiver, DC/DC module?, LDOs?, but I'm really curious about the usage of those feedthrough capacitors.

author: Kean
author_rank: Unknown
timestamp: May 28, 2018, 01:24:33 pm
content: Quote from: gamalot on May 28, 2018, 01:17:26 pmIIRC, the feedthrough caps (and most of the load cell interface components) are as per the ADS1232 datasheet/reference design.Other than the ADS1232, there is an ATmega328, MAX3232, & LM75.Power input is nominally 12V DC, into a 6.5V switcher, then a pair of low noise 5V LDOs (analog & digital).

author: gamalot
author_rank: Super Contributor
timestamp: May 28, 2018, 01:43:38 pm
content: Quote from: Kean on May 28, 2018, 01:24:33 pmThanks for your reply. So the SOP8 chip is LM75 not an EEPROM, there is internal EEPROM in ATmega328.I still feel like that the use of feedthrough capacitors is a little overkill here, they are expensive.BTW, I'm from China, we don't even use ADS1232 anymore, we use HX710/711, very very cheap.(I recently worked on my electronic scale, picture attched)

author: Kean
author_rank: Unknown
timestamp: May 28, 2018, 02:27:32 pm
content: Quote from: gamalot on May 28, 2018, 01:43:38 pmYup. Places where penguins nest tend to have extreme temperatures, so I thought it would be useful to include the LM75. I don't think I ended up using it for compensation, and it isn't logged at the penguin weighbridges. I think I do log it at another (wallaby) weighbridge installation in Queensland.The only need for the EEPROM is calibration constants and basic settings, which change pretty rarely. Possibly once a week at most, if the researchers follow procedure and there is any noticable drift.QuoteSure, but I wasn't building these in any significant volume, and I believe I already had the same feedthrough caps left over from another design.This was actually an urgent redesign to replace failed boards using a 20 y.o. design where the parts were no longer available. I wanted something that worked as well as possible first go!I developed two firmwares - one done the way I would do it, and one that was backwards compatible with the historical protocol in use and its idiosyncrasiesWhen I did noise tests, the 24 bit ADC gave me 19 effective bits which I was perfectly happy with. Better than the original design, and we only needed 1g resolution.QuoteI've used the HX711, and contributed to the Arduino library at the time. As simple as it was, I didn't like it one bit.Actually, V2 of the above design used the HX711. I assembled it quickly over a weekend as a stop-gap. It gave occasional whacko data which caused some grief, and required filtering. Could have been an implentation issue, but I wanted something better documented and that I felt I (and others) could trust. For something being used in scientific research, I wasn't going to risk using a poorly specified part.Back on topic...These have been in the field for about 3 years, suffering widely varying temperatures (not Antartic conditions, but unprotected coastal Australia), as well as costal salt air, and even full submersion after heavy rain. Running constantly with no reported problems. Knock wood...

author: jmw
author_rank: Frequent Contributor
timestamp: June 14, 2018, 08:35:07 pm
content: Follow-up: this is what I did. Countersunk M3 screws into the bottom and will put F-F hex nut spacers on top that will mount the PCB. I drilled the box instead of the lid since that will make it easier to get the connections done before assembly.