In pursuit of small USB MCU - Page 1
Forum section: electronics / general

author: blueskull
author_rank: Unknown
timestamp: October 18, 2018, 03:31:03 pm
content: Is there a small (WLCSP, <=3mm*3mm) USB MCU that has good library support (USB UART w/flow control) and is available in actually manufacturable package (pitch>=0.5mm)?I came across SAML22, but its 0.4mm package is a huge PITA. PSoC5LP is a good option, but it's way too big, and way to over powered and expensive.I don't need a ton of pins. Something like EFU8UB10F16-QFN16 is perfect, and it also integrates a voltage regulator, which is much needed, only if it's in WLCSP package.I'm open to any MCU architecture -- be it 8051, AVR, PIC, MSP430 or any ARM. If he library and compiler is good, I'm even open to 4-bitters.Any ideas?

author: janekm
author_rank: Unknown
timestamp: October 18, 2018, 04:17:29 pm
content: Do you have an upper size limit? I did see a few parts in 5x5mm QFN packages which is more or less what you'd get with a 0.5mm BGA...0.4mm WLCSP is fairly standard now so most modern assembly houses should be able to handle it. Usually for small packages the pinout is designed to put crucial pads close to the edge so you don't need any crazy via specs.

author: technix
author_rank: Super Contributor
timestamp: October 18, 2018, 04:48:39 pm
content: If you are okay with 5x5mm there is STM32F042G4U6/G6U6 in 5mm QFN package. The STM32F042T6Y6 is smaller but that is a 0.4mm WLCSP.As of library, I have been working HAL-less on STM32 for quite a while. You can fork my DAP42 code as a start point since it has boot code and USB all in place and ready.

author: chickenHeadKnob
author_rank: Super Contributor
timestamp: October 18, 2018, 05:19:12 pm
content: Nuvaton makes a USB series (usb 2.0 fullspeed) cortex m0 in qfn 5x5 mm. They have internal high accuracy resonator.https://www.nuvoton.com/hq/products/microcontrollers/arm-cortex-m0-mcus/nuc120-122-123-220-usb-series/?__locale=en

author: JPortici
author_rank: Super Contributor
timestamp: October 18, 2018, 05:22:16 pm
content: while we're throwing bigger parts, PIC16LF1455 or ATSAMD21 come in 4x4mm, 0.65mm and 0.5mm pitch QFN

author: technix
author_rank: Super Contributor
timestamp: October 18, 2018, 05:33:12 pm
content: Quote from: blueskull on October 18, 2018, 05:31:11 pmWhat might that contender be?

author: JPortici
author_rank: Super Contributor
timestamp: October 18, 2018, 10:19:48 pm
content: me shivers.I know you have access to a reflow over and anything.. but in the past i had to make some prototypes with a radio chip that had that same package, a gigantic PITA

author: PCB.Wiz
author_rank: Super Contributor
timestamp: October 18, 2018, 10:54:18 pm
content: Quote from: blueskull on October 18, 2018, 05:31:11 pmSiLabs also have EFM32HG series showing in 36-CSP (3.02x2.89), so that's a smaller routed-PCB footprint.

author: SiliconWizard
author_rank: Super Contributor
timestamp: October 18, 2018, 11:35:01 pm
content: The EFM32 CSP36 has a pitch of 0.4 mm if I'm not mistaken. So that's a pitch blueskull doesn't seem to like much.Other MCUs with CSP-type packages and 0.4 mm pitch:The STM32L082 or L052, 2.6x2.9ATSAML22NXP KL26 2.37x2.46 (ex: MKL26Z128CAL4R) but you won't like the 0.35 mm pitch.Finding an MCU supporting USB and coming in a CSP package <= 3x3 and a pitch >= 0.5 mm is no small feat. Can't find any so far.Yes SiLab's QFN-20 package seems to be the best fit so far, provided that the limited number of IOs is no problem.Frankly, as for me, I would prefer using a 0.4 mm-pitch BGA/CSP-type package rather than a 0.5 mm-pitch QFN for automated assembly. Any day. At least on the manufacturing point of view. YMMV.Now if you're hand-assembling, that's another story.They can be more of a hassle to route on PCB obviously though.

author: wraper
author_rank: Unknown
timestamp: October 19, 2018, 12:00:24 am
content: Quote from: blueskull on October 18, 2018, 03:31:03 pmYou misspelled the part, it's EFM8UB10F16G-C-QFN20. Why would you need WLCSP when this MCU is 3x3mm in size as you asked, 0.5 mm pin pitch

author: ataradov
author_rank: Super Contributor
timestamp: October 19, 2018, 02:55:16 am
content: SAM D11 is 2mm x 2.4 mm, but still 0.4 mm pitch. If nothing else, it has fewer pins you need to worry about.

author: ataradov
author_rank: Super Contributor
timestamp: October 19, 2018, 04:52:59 am
content: Quote from: blueskull on October 19, 2018, 04:48:19 amNot a problem at all. All I/O pins are inputs by default.I think there is an official VCP implementation, but if you don't like bloat, then I have a small one herehttps://github.com/ataradov/vcp.

author: ataradov
author_rank: Super Contributor
timestamp: October 19, 2018, 05:06:25 am
content: Quote from: blueskull on October 19, 2018, 04:58:37 amThere are hookups for line status signals, which are used to control a configurable "status" pin. This was done to support a call to the Arduino-style bootloader.But it is no a full flow control implementation. I frankly got confused by the USB CDC implementation of the line status signals, and gave up on implementing them. I don't think it is hard to add this, I just did not have any motivation to do so.

author: wraper
author_rank: Unknown
timestamp: October 19, 2018, 08:31:52 am
content: Quote from: blueskull on October 19, 2018, 03:01:44 amConsidering this package has 4 LGA style pads on the corners, you could just use LGA footprint style for all of them. There is not actual need to expose pads them to outside. Actually I had troubles with this package because when I several times ordered paste stencils and explicitly asked to not modify center pad, fabs reduced center pad area anyway. And then solder got squeezed out from those corner pads. I have doubts if this package should be called QFN to begin with.

author: wraper
author_rank: Unknown
timestamp: October 20, 2018, 06:47:16 am
content: Quote from: blueskull on October 20, 2018, 04:52:20 amBut one of those corner pads is required to be used as it's a power pin. Therefore when soldering you should treat it more like LGA rather than QFN (must be somewhat elevated above PCB). Otherwise solder might get squeezed out. Just like when those stupid fabs treated it like usual QFN when producing my stencils. If not extending pads, IMO it should work just like when soldering LGA.