Why aren’t bypass capacitors built into IC’s? - Page 1
Forum section: electronics / beginners

author: danielbriggs
author_rank: Regular Contributor
timestamp: June 03, 2018, 08:31:43 pm
content: Hi all,After watching Dave’s excellent couple of videos about bypass capacitors... I was left wondering the question “why aren’t bypass / decoupling capacitors built into the IC package if they’re needed for optimal operation?They’d be super close to the die, so that would be a good thing?Is it just space and cost issues preventing this? Or are there any better reasons for this not being the norm.Thanks,Daniel

author: coppice
author_rank: Super Contributor
timestamp: June 03, 2018, 08:41:45 pm
content: It is unusual to put decoupling caps inside the IC package, but decoupling caps on the outside of the package are fairly common for high performance parts.

author: Zero999
author_rank: Super Contributor
timestamp: June 03, 2018, 08:43:37 pm
content: I believe this question has been asked here before, but can't find it.It's because there isn't space in the package and it's just cheaper to put them in externally.EDIT:It's been asked on Stack Exchange, which Google shows on the first hit. I don't know about here.https://www.google.com/search?q=Why+aren%E2%80%99t+bypass+capacitors+built+into+IC%E2%80%99s%3F&ie=utf-8&oe=utf-8&client=firefox-b

author: David Hess
author_rank: Super Contributor
timestamp: June 03, 2018, 10:59:44 pm
content: Sometimes decoupling or other capacitors are added inside the IC package either on the IC or on the lead frame but it is usually more economical to require them externally.

author: KE5FX
author_rank: Super Contributor
timestamp: June 03, 2018, 11:47:15 pm
content: It's becoming more common to see caps inside IC packages. They are not always mentioned in the data sheet, but they can be 'found' if you look.E.g., here's a nice shot of a switching regulator (LT8650S) with both input- and output-side bypassing:And a clock synthesizer chip (LMK61E2):They are present in FPGAs too in some cases (Kintex for sure, and presumably Virtex). I wouldn't be surprised if they're used in SoCs and SDRAM parts as well.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2018, 12:33:03 am
content: FYI, the crossover point is around 100MHz and a few ohms.If the IC needs a supply impedance lower than that, or at a higher frequency than that, it has to be provided on package, or on die.If lower, then it can be provided on the PCB, to varying degrees of difficulty (worst case, say: multilayer board with stacks of alternating planes and tons of vias under a BGA?).Tim

author: amyk
author_rank: Super Contributor
timestamp: June 04, 2018, 02:39:22 am
content: Look at a relatively recent CPU from Intel or AMD, the bypass caps are very obvious.

author: ejeffrey
author_rank: Super Contributor
timestamp: June 04, 2018, 03:01:29 am
content: One reason is simply processing. You can't make a very large capacitor on a silicon chip compared to a multi-layer ceramic cap. You can add ceramic capacitors by bump bonding them to your chip or package, and that sort of thing is done when necessary but it adds extra fabrication steps. You can get more capacitance with higher ESL than on-chip capacitors. On the other hand, you typically get less capacitance with lower ESL than with capacitor mounted on the PCB. So on-package capacitors don't usually eliminate the need for PCB mounted capacitors, they typically just bridge the gap between what you can do on chip and on the PCB for applications where that is required.

author: helius
author_rank: Super Contributor
timestamp: June 04, 2018, 03:16:23 am
content: Many of the classic ceramic PGA packages have gold pads on top for bypass capacitors. They were mainly for insurance for extra noise margin etc. since in most cases they weren't used. As packages changed to epoxy and metal IHS, etc, the caps got relocated and can be found on the interposer, under the spreader.I see a comment above showing an xray with capacitors embedded in the package, which would be possible if they were soldered to the lead frame before epoxy injection. Placing a capacitor in the IC itself would require a dielectric compatible with the wafer fab process which I would think is much more daunting. You would also have to calculate leakage from the plates to other structures in the solid state die which could be a problem.example:visible under the spreader:

author: danielbriggs
author_rank: Regular Contributor
timestamp: June 04, 2018, 12:38:57 pm
content: Thanks everyone,Your replies are really interesting - thanks for taking the time to explain.All the best,Dan