Deciding on generic parts - Page 1
Forum section: electronics / general

author: cadr
author_rank: Contributor
timestamp: May 24, 2021, 01:58:07 am
content: Hello,I'm trying to design my first PCB. I'm using EasyEDA and want to get it made/assembled by JLCPCB.The thing I'm getting a bit held up on is choosing the passive components. There are *so* many choices for resistors and capacitors, and I don't really care all that much on the specifics for this specific design. Do you have a recommended way you go about narrowing down the field?(As an aside, the it is cool that the tool integrates the parts available, but am having trouble just doing "show me 10k resistors that are your basic/not extended parts". Have so far been going to their website, finding a part, then searching for that back in the tool.)Thanks!-Ben

author: Jackster
author_rank: Frequent Contributor
timestamp: May 24, 2021, 02:03:48 am
content: *sorts by price ascending*

author: cadr
author_rank: Contributor
timestamp: May 24, 2021, 03:09:00 am
content: Quote from: Jackster on May 24, 2021, 02:03:48 amYeah, if they gave me a way to do that, that would be my first go to.

author: phil from seattle
author_rank: Super Contributor
timestamp: May 24, 2021, 03:55:31 am
content: Quote from: cadr on May 24, 2021, 03:09:00 amThe JLCPCB (and LCSC) interface sucks beyond belief. But you are in luck -the jlcpcb component catalogworks great. It's a little quirky but once you figure it out, you will wonder how you managed witout it. Send the guy a couple of bucks as thanks.Anyway, look first for basic parts. That will narrow it down a lot. Then know what voltages and power you are dealing with. (how much current and voltage through your resistor, pick a package that can handle it, for example). Then pick the cheapest fit.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: May 24, 2021, 02:26:06 pm
content: Keep in mind the value precision range and any failure/temp ratings for components.I assemble my own boards, and they are high reliability boards, so I use automotive grade 1% resistors/components. They cost about 1.2 cents each buying by the full reel. So if I went with cheap resistors, the cheapest comparable I found on Mouser just now is .2 cents per each, by the reel.Based on 200 resistors per board, I'm paying an extra $2 to use the high quality 1% automotive rated resistors vs 5% un-rated resistors.Same kind of deal with ceramic caps and all other components.It completely depends on your end usage and to what price point you are aiming at for if this makes a difference or not, but is something to keep in mind.

author: cadr
author_rank: Contributor
timestamp: May 25, 2021, 05:59:11 pm
content: Quote from: phil from seattle on May 24, 2021, 03:55:31 amOh, wow! That will help a lot. Thank you!

author: Mangozac
author_rank: Frequent Contributor
timestamp: May 26, 2021, 12:11:29 am
content: Quote from: Jackster on May 24, 2021, 02:03:48 amThen cross-reference with *sort by stock descending*

author: phil from seattle
author_rank: Super Contributor
timestamp: May 26, 2021, 04:32:44 am
content: Quote from: Mangozac on May 26, 2021, 12:11:29 amThis is worth expanding on. If I have a couple of choices of devices to use in a design, I will often look at the JLCPCB and LCSC sites to see which ones are more plentiful. I often select the one that has the most stock, betting that it is more likely not to be out of stock in the future. Not a guarantee but in today's environment of shortages it is better than just hoping.

author: Mangozac
author_rank: Frequent Contributor
timestamp: May 26, 2021, 05:31:30 am
content: Quote from: phil from seattle on May 26, 2021, 04:32:44 amI'll typically search for a part on Digikey because it has the best parametric search out there. Once I've found a candidate I'll then plug it into Octopart and see what stock other distributors are carrying. I've found that to be a generally reliable way to work but of course there's still edge cases (component gets end-of-life'd, or the current global shortage where everything is going out of stock in the blink of an eye).

author: Styno
author_rank: Regular Contributor
timestamp: May 26, 2021, 09:39:20 am
content: In addition to of what some others have already suggested we try to keep a few rules that makes it easy to use the same components in many applications, unless specified otherwise. E.g.- resistors:All 1% (so they are also usable in voltage dividers for e.g. voltage regulator feedback)- capacitors:<100nF: 0603 X7R 100V<=10uF: 0603 X7R 16V<100uF: 0805 X5R 10V>=100uF: 1206 X5R 6.3VThis extends to mosfets and other stuff and in general these are of the 'better-in-class' type. For some application these 'standard' components may be overspecified but this enables less inventory and bookkeeping, easily compensating the extra cost for each component in smaller outfits like ours.

author: Mangozac
author_rank: Frequent Contributor
timestamp: May 26, 2021, 11:21:44 am
content: Quote from: Styno on May 26, 2021, 09:39:20 amThat's good advice - we have done the same. Much of the time there's $0.0002 different between a 1% and 5% chip resistor, so unless you're doing massive volume, extremely price sensitive stuff it's just way easier to stick with 1%.

author: Rat_Patrol
author_rank: Frequent Contributor
timestamp: May 26, 2021, 02:24:46 pm
content: Quote from: Styno on May 26, 2021, 09:39:20 amGreat advise, we operate the same way.

author: Siwastaja
author_rank: Super Contributor
timestamp: May 27, 2021, 01:20:19 pm
content: Also try to standardize on a size, only use different size for a very good / specific reason. This means, use the smallest size you can really handle without struggle.Usually for us mere mortals who can't handle 01005 parts, it's either 0402 or 0603 depending on who you ask, or some dinosaur might prefer 0805 by default. For me, definitely 0402, it makes a real difference compared to 0603 let alone 0805 when you need 20 caps and resistors around a 0.5mm pitch QFP144 and want to keep them close, not route miles worth of traces to work your way around large passives.Some say hey, you have excess PCB area left free, you could have used larger components to make prototyping easier. No shit Sherlock, but then you would be ordering different component sizes and would need to swap the packages during design phase all the time depending on how the PCB routing happens to go. Unused PCB real estate is great, it's room for future opportunity (add that missing feature you forgot, or miniaturize down).Agree that just use 1% resistors, cost saving to go into 5% is negligible and requires extra work by reducing amount of part reuse you can do. For the same reason, it's a good idea to use slightly "better" parts, for example default on X7R even if X5R does the job for many/most designs. Unless your volume is in tens of thousands, prioritize patterns that save your time.

author: phil from seattle
author_rank: Super Contributor
timestamp: May 27, 2021, 01:57:11 pm
content: Quote from: Siwastaja on May 27, 2021, 01:20:19 pmBefore the rise of services like JLCPCB, I would have agreed with you. But now you can use any size of jelly bean passives you want when getting boards assembled with no need to stock them in house. I find 805's the easiest to rework on my prototypes so I use them a lot. Also, not cramming them together as close as possible helps. For me, the big issue is getting a soldering iron in to change a component. Typically, I will do an 805 design for my first turn leaving room for rework. When I am convinced the design is solid, I do a shrink to 603/402 - all made by an assembly house.

author: cadr
author_rank: Contributor
timestamp: May 27, 2021, 05:02:02 pm
content: Thanks everyone for the advice!I think the standardizing on a type of part (like 1% tolerance) will help me a lot in speeding up the process.Cheers!

author: Siwastaja
author_rank: Super Contributor
timestamp: May 29, 2021, 12:06:46 pm
content: Quote from: phil from seattle on May 27, 2021, 01:57:11 pmServiceslike JLCPCB- are there really others? Mention one. Do they work? Are they future-proof? Do they guarantee the availability of parts?To me, it seems JLCPCB hasn't yet changed how the industry works. We'll see maybe in 5 years. In the meantime, I apply all the old methods; I still have to build the prototypes, for which I still have to order components as the BOM says. I still need to manage the BOM.I think the current component crisis has shown you can't trust JIT component ordering or sourcing by the PCBA company. It never was reliable, we just took the chances. So stocking BOM for the next 6 months is more important than ever.And, I still operate by requesting quotes from PCBA companies. Right now we are using a small local one for quick turn-around. BOM optimization is as critical as it always has been.As it stands, JLCPCB's new concept is indeed exceptional but with experience from any engineering field, it's a recipe for disaster to totally change your workflow based on a new service available from a single supplier and completely trust their workflow, however good it seems.And I hope it all changes in the way of JLCPCB, but I don't think we are there yet.Besides, JLCPCB is really useless for most serious builds because the component selection is so severely limited and their process won't allow sending them parts. When I think about it, I could have manufactured maybe ONE out of some dozen boards I have designed during last few years there. On the other hand, I have sent special components (not available through distributors) to PCBWay for assembly "the classic way" no problem.And having to mentally maintain two separate workflows doesn't make a lot of sense. I do understand though that many former beginners, former "buy adafruit module" designers who would have never ordered a PCBA with the classic process are now using JLCPCB's service, mostly due to price. For them, it's a single workflow and might be a stepping stone into going "real" PCBA ordering.BTW a 0402 is often easier to desolder for component value change than a 0805; apply a blob of solder to your typical chisel tip, touch both pads at once from the side, the 0402 part sticks to the iron tip. (Don't try to reuse the part though, obviously.)

author: phil from seattle
author_rank: Super Contributor
timestamp: May 30, 2021, 04:25:26 pm
content: Perhaps not at the level of JLCPCB but there are more and more services that you hand them a BoM, gerbers and they build. There was another posted about in this section recently - limited to 5-10 boards though I am sure would make more if you asked. My point is you don't need to keep broad stocks of components.The JLCPCB catalog is pretty big. Granted, if you need a specialized chip they don't carry forget it. But if you can design within their catalog, it is pretty good. I keep hearing - you can't build anything professional with them (or similar words). I've sold over 500 boards made by them. My customers are a pretty happy lot. I've only had to refund 1 customer and that was because the delivery service in his country couldn't get it to him. The board originally used a number of components not in their catalog but I did a redesign to fit and got good results.I don't want to look like a shill for them but they have been improving their processes in the year+ I have been using them. Their solder mask seems a lot tougher. HASL is definitely better. Their alignment of components is very much improved. My last batch (received last week) had every component perfectly placed and aligned - not even slight (but acceptable) misalignments of components.As to reworking really small stuff - you are clearly good at it but some of us have limitations that make it hard. Should we just quit trying because we can't come close to your level? I have a system that works for me.

author: TimFox
author_rank: Super Contributor
timestamp: May 30, 2021, 06:15:44 pm
content: I would suggest C0G instead of X7R for low-capacitance 0603 parts, perhaps below 1 nF. The dissipation factor and voltage stability are far better for C0G, but they become expensive at high values.

author: cadr
author_rank: Contributor
timestamp: May 30, 2021, 10:19:18 pm
content: I just want to say thanks again for the advice, and especially that parametric search. Made things *so* much faster. (And I did buy that guy a coffee)I've now designed my first PCB and put it up for review here:https://www.eevblog.com/forum/eda/beginner-pcb-review-request-rf-amp/

author: peter-h
author_rank: Super Contributor
timestamp: May 31, 2021, 06:50:35 am
content: I would use 0805 components unless you need small size, in which case use 0603. Below that the resistors are unmarked... 0805 remains available because not many use it.Also use "commodity parts" whenever possible e.g. LM358 op-amp rather than something fancy from Linear Technology or Maxim who will screw you on pricing down the road.I use JLCPCB for prototypes but could never use them for 1k+ production. Their communication is very poor; their website messaging box only just about works (I think they google translate the English) and problems are hard to resolve. On the last order, 1 panel of 10, I got 10 panels of 10. Well it was still cheapBut one cannot work with such a company for serious stuff. China is hard enough to work with anyway, with companies disappearing regularly, and comms being difficult and sometimes quite arrogant. Tooling disappearing, etc... I now buy just PCBs and cables from there; pulled out all finished product assembly.

author: cadr
author_rank: Contributor
timestamp: May 31, 2021, 02:39:36 pm
content: Quote from: peter-h on May 31, 2021, 06:50:35 amOh, I'm just using them for learning and hobby stuff for myself.

author: tszaboo
author_rank: Super Contributor
timestamp: May 31, 2021, 03:27:06 pm
content: Quote from: Jackster on May 24, 2021, 02:03:48 amHaha, perfect way to make boards that are never made. This is 2021.Sort by "In stock quantity". Pray to favorite deity.Dont try to standardize on JLCPCB parts. They just try to sell you whatever they happen to have at the moment. Completely unreliable for businesses where you want to have the same part every time.

author: peter-h
author_rank: Super Contributor
timestamp: May 31, 2021, 04:28:22 pm
content: That comment would be true for any scenario where a chinese company is assembling your product with parts which they are sourcing

author: Siwastaja
author_rank: Super Contributor
timestamp: June 03, 2021, 11:38:34 am
content: Quote from: phil from seattle on May 30, 2021, 04:25:26 pmSounds like a classic CM process to me. When you place the order, they start ordering bulk of the components. The more lines on BOM, the more expensive it gets. The more BOM lines, the higher the risk of some part being out of stock on their supply chain. So definitely optimize the BOM, like always.Now it's more important than ever to stock all key parts yourself and send them to your CM.

author: cadr
author_rank: Contributor
timestamp: June 10, 2021, 04:47:55 pm
content: Hey all!Thanks for the advice. Got the boards in and they look good. Need to take more measurements, but they are amplifying, which is a great result for my first PCBThe github library was *so useful*.Pictures, etc:http://kc9dlm.blogspot.com/2021/06/babys-first-pcb.html