Unecessary use of 0402 and 0201 capacitors and resistors in SMPS circuit - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: January 17, 2021, 03:39:11 pm
content: Hi,Ive just been presented an 120W offline flyback SMPS from a contractor.Some of the resistors and capacitors in the signal circuitry of the power supply are unnecessarily using 0201 and 0402 resistors and capacitors. Even though there is plenty of room for these to be 0603. For example, some of the compensation capacitors are 0201 or 0402, etc.In my experience, 0201 and 0402 resistors and capacitors are more likely to suffer “head on pillow” or “tombstoning”, or dry joint…when compared to 0603 size.Would you agree with me? This is bad news and they need changing to 0603?

author: fourtytwo42
author_rank: Super Contributor
timestamp: January 17, 2021, 07:39:50 pm
content: Did Opampsmoker get cremated then

author: rfclown
author_rank: Frequent Contributor
timestamp: January 17, 2021, 08:29:10 pm
content: I work for US companies that do mostly low and medium volume and use CMs for PCB manufacture. 0201 do not present a problem over larger sizes for the CMs I've worked with (China and US). I personally use 0402 unless there is a reason not to, but mainly for my convenience. I can work with 0402 fine for rework (I hate working with 0201), I have lots of sample kits of everything in 0402, and parts seem to be easier to find in 0402. I use 0201 only when space demands it, or for RF parasitic reasons. I only use 0603 and larger for power in resistors and voltage in capacitors.Alot of things I've seen with 0201 where it isn't necessary is because it was picked up from a reference design and they didn't want to change anything. CMs have no problem with them. They are just a pain in the butt to rework (for old me at least). I settled on 0402 as my goto about 10 years ago, and at 59yo, they are still my prefered size. I do need a microscope, good tweezers and good iron; but I need those for larger sizes also at my age.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 18, 2021, 05:10:54 pm
content: If you have tombstone issues with 0402 size, you need to look at what's wrong with the process. If the process is right, the yield is basically 100% and can't be much improved by using 0603 size.And obviously, professional designers and professional contract manufacturers do have a working process. Basically the designer's part is to draw proper footprint with correct paste mask openings, and correct copper layer with proper thermal reliefs; CM's job is to make sure paste print quality is good, placement accuracy is OK, reflow profile works, and so on.Those who can't do this, die out. Some struggle for some time, keep doing sub-standard production and sidestepping the actual issue by arbitrary limitations like "use 0603 or bigger". Yet the issue is, going for 0603 increases yield only so much. If you have problems with 0402, likely 0603 isn't trouble-free either.Because there is absolutely nothing wrong with 0402, there doesn't need to be any specific reason to use it "instead of" 0603. It's like a "have you stopped beating your wife" question.Choosing biggest possible part for each spot separately is unnecessary extra design work (while laying out, you would need to change the part number, verify stock, etc. multiple times during the layout work; even small changes would reflect back into the complete supply chain), requires larger and more diverse BOM, requires stocking more parts if multiple different designs are produced, and so on.By using smallest non-problematic by default, ease is maximized, and both risk and cost minimized. Often, this means:0603 for totally ancient "old fart" design houses, those who use hobby-level machines (sub Chinese $5000 DIY P&P kits...), etc., Basically "stuck in early 1990".0402 for low-cost, old process,0201 for modern day fabs.I default to 0402 which is also acceptable in prototyping although 0603 might give some insignificant time savings in hand prototyping but that isn't a significant enough percentage to be meaningful for me.

author: ejeffrey
author_rank: Super Contributor
timestamp: January 18, 2021, 05:47:36 pm
content: Yeah, I always use 0402 unless something else is required. Availability is good, they are easy enough to rework on prototypes if you have good magnification and I have never had significant yield problems with them. I would certainly never go into a working design and convert them to something larger.I haven't used 0201 much because I don't work on heavily space constrained products and rework is important to me. But I wouldn't avoid it for manufacturing reasons. If your manufacturer can't assemble them find a new one.

author: coppercone2
author_rank: Super Contributor
timestamp: January 18, 2021, 08:43:00 pm
content: your old machines don't have what it takes to do those parts and he needs to change it. i actually agree with you that putting unnecessarily small parts is not ideal. Just say your PCB guy said he can't do it. Say you got a good deal on the PnP and it does not have enough placement resolution to do those without unnecessary expenisve QC, wheras with bigger parts your really confident in it.

author: nctnico
author_rank: Super Contributor
timestamp: January 18, 2021, 10:14:36 pm
content: Quote from: Siwastaja on January 18, 2021, 05:10:54 pmThat is true to some point. The bigger the parts the less likely you run into trouble and you don't have to do much work to tune the soldering process very precisely. Also (automated) optical inspection is easier for bigger parts. I agree though that it is good to standarise on a specific size of components and don't mix too much. Using 0603, 0402 and 0201 in a switchmode PSU while there is enough space is madness. I'd go for 0603 since these are more likely to work with wave soldering compared to 0402.

author: rfclown
author_rank: Frequent Contributor
timestamp: January 18, 2021, 10:38:46 pm
content: Quote from: nctnico on January 18, 2021, 10:14:36 pmI had no idea that you could wave solder SMT partsMy first job in 1986 used through hole parts on a two sided board and it was wave soldered. The product that replaced that was SMT, and I haven't seen wave soldering since. Or do you wave solder after IR reflow? I'm showing ignorance here.

author: nctnico
author_rank: Super Contributor
timestamp: January 18, 2021, 11:21:57 pm
content: You can wave solder SMT parts for sure. Even 0.5mm pitch QFP parts can be wave soldered (if the footprint has solder thieves). The first stage is to glue them to the board and then go through the wave soldering machine. This is often used when there is a mix of through hole and SMT components (where the SMT components are placed on the solder side).

author: Siwastaja
author_rank: Super Contributor
timestamp: January 19, 2021, 07:22:35 am
content: Wave soldered SMT parts are quite common in very low-cost devices where most (or say, half) of the parts are TH. The SMT parts are glued on the bottom (solder) side; wave solder then takes care of both the SMT and TH parts at the same time. 0603 is a typical minimum size in such process and that is of course already quite miniature compared to the TH parts!

author: brabus
author_rank: Frequent Contributor
timestamp: January 19, 2021, 08:20:29 am
content: Probably it has to do with the supply chain. We had major issues in 2018 with 0805 and 0603 parts in Europe.

author: JohnG
author_rank: Frequent Contributor
timestamp: January 19, 2021, 05:53:08 pm
content: If you look at the higher density power modules, especially anything under 100V, you will see a lot of 0402s. These power modules command a premium for higher power density. There is a lot of stuff that does not process power directly, namely control and sensing systems. When you are doing everything you can to get more power processing parts and fewer control and signal parts, and as much copper as you can squeeze into the design, size matters. You only see bigger parts where they have to hand the power, or for power bus capacitors where you need a certain volume of capacitance, no matter how you divvy it up.In addition:Manufacturers have no problem with 0402s, and many have no problems with 0201s on up to 2 oz (70um) copper.They are reliable.PCB area isn't free.If the smaller parts work, I can think of zero reason to go larger if you make any volume at all. If you are making prototypes, then you can use bigger parts if you want.Cheers,John

author: Siwastaja
author_rank: Super Contributor
timestamp: January 21, 2021, 07:16:52 pm
content: Forseriousminiaturization, expect to see 01005 parts. But that adds cost and/or requires minimum volumes and using correct contract manufacturers.0402 was state-of-the-art miniaturization somewhere early 1990's. The good side is, every even remotely modern-day CM can do it, including lowest cost Chinese fabs, and most low-cost prototyping shops. A "shaky old man and his wife and son limited, est. 1970" may not be able to, if they didn't keep up with the times.But passive size isn't everything. For example, in power modules as mentioned, you have certain sizes of MOSFETs and ICs which require the die area for power. Even if they optimized for minimum packaging size, the dies are approximately the size of, say, a 0603 or 0805 part, so if you need to add five passives around it, going below 0201 gives diminishing returns.

author: Gyro
author_rank: Super Contributor
timestamp: January 21, 2021, 09:34:42 pm
content: Quote from: Siwastaja on January 19, 2021, 07:22:35 amScary to say now, but we were wave soldering 0603 parts on side 2 of multilayer boards 40 years ago, with IR reflow actives on side 1.Philips were doing the same thing, with 0603 passives and IC actives and through hole on their consumer products at the same time. Theirs were on single sidedSRBPboards too! ICs mounted diagonally, with solder thieves at the ends are a giveaway.

author: f4eru
author_rank: Super Contributor
timestamp: January 22, 2021, 09:25:48 am
content: Yes,PSUs are often, even today, manufactured in Wave soldered SMD process.This process is easier to control with bigger SMD components.

author: floobydust
author_rank: Super Contributor
timestamp: January 22, 2021, 07:41:09 pm
content: You have to be careful and do the power dissipation calcs for each resistor. 0201 rated max. 50mW and 1/2 that for good practice or high ambient temps. It's easy to miss that.I've never tried this 0402 "special" footprint to stop tombstoning:https://www.worthingtonassembly.com/blog/2015/4/17/working-with-a-manufacturer-the-best-0402-footprint

author: ejeffrey
author_rank: Super Contributor
timestamp: January 22, 2021, 07:59:06 pm
content: There is nothing special about that footprint it is just a standard 0402 footprint. Which they clearly state on that page. The claim is that eagle ships with a bad 0402 footprint. I haven't really looked at eagle footprints in a long time, but I coudl totally believe that.

author: Siwastaja
author_rank: Super Contributor
timestamp: January 23, 2021, 09:32:14 am
content: Altium also came (or maybe still comes?) with some completely broken SMD footprints, including a 0402 footprint which kinda works but tombstones easily, and 1210 capacitor footprint which has pads so small and so far away that you simply can't succesfully reflow a 1210 cap on that footprint; with iron, it's possible to make solder bridges.Go figure.IPC publishes the recommended standard footprints, it's utterly trivial to just follow them, and you couldthinkprofessional EDA software vendors would use them for libraries which ship with the products.One behind that link is very close to the IPC footprint.The recommendation of always drawing your own footprints, maintaining your own libraries, forallparts, is really a good one. There likely are proper libraries available, but how do you know that, given that most are shit, and stupid amateurs are seemingly happy with them so it's hard to find discussion? Since you need to be able to draw partsanyway, why not just get good at doing that, in which case drawing all footprints isn't too daunting.Broken EDA library isn't a good reason not to use 0402 parts, because it's trivial to fix the actual root cause (wrong footprint). With 0402 parts, you get so many advantages compared to artificially staying with 0603 and bigger, it'sreallyworth the extra 5 minutes of correcting the footprint once.