My bill of materials tries to break my spirit... - Page 1
Forum section: electronics / projects

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 17, 2015, 10:55:43 pm
content: Hello!At first i wanted to say "It is my first big project" but i have done bigger stuff then this.But everyone except 3 of my boards i etched and drilled myself, only 3 i had made.And those three had a bunch of components on them but only 4 different values...Yesterday i ordered a panel with 12 different boards (2 each, 24 total), milled, solder mask, silkscreen... all the fancy stuff.Getting the panel right, the routing on all 12 boards, figuring out how each chip works, searching for the right parts...In all it took me about a month, maybe more.I printed the boards on paper, made sure the footprints were right, checked for clearence. Placed crucial components on the "printed paper boards" and dit everything to minimise the possible snafus. I am certain i missed something but that is ok.But today i put all the parts in a sheet, checked what i had and what had to be ordered and nearly lost it... -_-I only have to order passives, for the most part, but there is so much on these boards...One 10Ohm resistor here, a 10pF capacitor there, another 10µF here, another two 60,4K resistors... the list goes on.Odd values the datasheet demandet or the calculations predicted, from the start i used the same component values whenever possible.I have to split the order between two different retailers because it would cost twice as much ordering just from one retailer.Tomorrow i will have to face the music, i dread the moment i see the final value on the invoice.My piggy bank will most likely be sacrificed...

author: Whales
author_rank: Super Contributor
timestamp: February 17, 2015, 11:19:51 pm
content: I tend to work my designs in reverse: what do I have already lying around that I can make things with? It saves me a lot of money and I learndifferentthings. Keep in mind however I'm a cheap unemployed student and many members of this forum wouldscoffat my designsDid you attempt any costing down on your board? If you have not yet ordered your parts, please post pictures of your PCB + schematic (if you have designed your board this way) and we might be able to give you a hand.

author: max_torque
author_rank: Super Contributor
timestamp: February 18, 2015, 12:17:35 am
content: In commercial production electronics a lot of the ART is knowing how much you can "barstardise" any given design to reduce the cost of the BOM and simplify it's manufacture! The trick is that your device needs to JUUUUSSST work, and no more, under whatever conditions it is experiencing.

author: Whales
author_rank: Super Contributor
timestamp: February 18, 2015, 12:27:40 am
content: Quote from: max_torque on February 18, 2015, 12:17:35 amThis is very important, but as with everything some people like to take it too comedic levels.Need a half-bridge rectifier? WellI guess we can afford a couple of those...... this is too damn expensive! Shove some FR302's in!Anyway, we might be able to help you still if you have not yet ordered your parts.

author: Siwastaja
author_rank: Super Contributor
timestamp: February 18, 2015, 02:23:47 am
content: If you plan doing small-scale electronics production in the future (i.e., this is not one-off project), and do relatively small batches (hundreds, not thousands of components of each type), get yourself a decent set of basic passives in full reels. For exampe, a reel of resistors - 15000 pcs at 0402 footprint - costs something like 3 EUR. Invest something like 200 EUR once and you will havemostpassives in nice, big reels and save a lot of costandtime when you only need to order "special" parts. It's much easier and less stressful to order when you have 20 items instead of 50, even if most of the money goes into those 20.Also, having those physical components gives you the mindset to really try to use them as much as possible.Try to standardize in one footprint size. I use mainly 0603, but have some 0402 (100 nF decoupling near the power pins of fine-pitched QFP, and a few resistor sizes for IO series resistors / pull ups / rc filters, like 100 ohm, 1K and 4k7)Note, sometimes it's cheaper to get a "strange" resistor value. For example, when I stocked some "basic" values from Farnell/element14, 0402 4k7 were 6 EUR for 15000 pcs, but only 3 EUR when I opted for 4k99 instead. So as a general advice, shop around. And it's nice when you only need to do it once, hence get full reels and use some time finding the good deals.

author: PeterFW
author_rank: Frequent Contributor
timestamp: February 18, 2015, 05:24:53 pm
content: Hello and thanks everybody for the reply!Quote from: Siwastaja on February 18, 2015, 02:23:47 amSome day it would be nice to build something that someone else likes to buy.But this stuff i do just for my own plasure with no budget to speak of -_-So i only need a couple of each.Except for the odd capacitor and resistor i only use 0805 parts, handsoldering those is a little bit more relaxed then 0603.Quote from: Whales on February 17, 2015, 11:19:51 pmThere is not much room for parts and cost reduction.Those are a bunch of board to evaluate various chips and to see if my layout works as intendet. I do not want to put everything on one board and hope for the bestIf i try to fiddle with the component values, i can not tell what the failure is if it does not workQuote from: max_torque on February 18, 2015, 12:17:35 amYea... i tend to over think and over do it sometimes... -_-I just gave the whole thing a good once over, and managed to reduce the different values needet by a third. That i can manage.This is what remains in the attached file, the main problem is, mostly it is only 2-4 of each, that makes it hard to just buy a few 100 each.I have done that in the past and still have components from 10 years ago i thought i might need some day.There is allways a part missing, every time i have to order something annyway.Because of that i do not want to "hoard" components anny more.

author: Whales
author_rank: Super Contributor
timestamp: February 18, 2015, 09:55:23 pm
content: From an uneducated look at the 'to buy' section of your BOM with a lot of guesses:SMD caps: If a few of these types are on bypass duty, take the a guess at a compromise value and use lots of that instead, even if it means doubling them up. The tiny bit of extra soldering is nothing compared to saving on parts complexity and ordering.You're using some largish-value 0805 SMD caps. If you find them expensive, don't be afraid to redesign/use other types.Cermet trimmers: If these are all for voltage dividers then try just one size and vary the surrounding resistors to suit.High wattage resistors: if you have lots of 1 ohm or similar 1/4W resistors lying around, bundle them together and twist the leads. They probably have less inductance than traditional wire-wound cement resistors anyway.You may have thought about these things already, I don't know. Also be wary of the 18650 holder: depending on whether or not your cells are protected they may provide a poor fit, depending on their design.I on the other amreally cheap:SMD ferrite bead: avoid the ferrite, just turn more wires or if it's low-freq shove an iron nail in.18650 holders: Solder wires onto a couple of small pieces of steel and use magnets to attach them to the battery terminals.Hopefully we've been of some help. None the less, make sure you enjoy what you're making

author: mzzj
author_rank: Super Contributor
timestamp: February 19, 2015, 06:40:36 am
content: Quote from: PeterFW on February 18, 2015, 05:24:53 pmMost of the stuff looks dirt cheap. To me the problem would be that it looks like the BOM is probably less than 65 euros and thus below the free shipping limit of the digikey. So need to order more stuff!You could probably use only one type of cap for example instead of these 3:10µF 16V SMD-0805 SMD-08054,7µF 16V SMD-0805 SMD-080522µF 16V SMD-1206 SMD-1206Use oversize 10uF cap instead of the 4,7uF and parallei 2 to replace the 22ufOr use only 4,7uf caps, parallei 2 or 4 as needed.