PIC16F18856 has centre metal pad? - Page 1
Forum section: electronics / general

author: ocset
author_rank: Super Contributor
timestamp: August 07, 2018, 01:45:09 pm
content: Hello, Do you know if the 6X6mm version of the PIC16F18856 has a centre metal pad? The datasheet doesn’t say. The recommended land pattern (page 664) has a centre pad, and it is a potential short circuit hazard to the pin pads.......so we will not follow the recommended land pattern if the actual IC itself doesn’t have a metal centre pad.PIC16F18856 datasheethttp://ww1.microchip.com/downloads/en/DeviceDoc/PIC16(L)F18856_76%20Data%20Sheet%20DS40001824C.pdf

author: wraper
author_rank: Unknown
timestamp: August 07, 2018, 01:58:29 pm
content: Quotepage 5, and then look at page 652-655 for package drawing and pad layout.

author: NorthGuy
author_rank: Super Contributor
timestamp: August 07, 2018, 02:00:15 pm
content: All the QFN packages of any PICs that I've ever seen had a central pad. The datasheet that I have for PIC16F18856 does shows the central pad on the drawing.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: August 07, 2018, 02:14:49 pm
content: Quote from: treez on August 07, 2018, 01:45:09 pmAre you seriously not able to navigate a datasheet to find a package drawing??? Who hired you?

author: andersm
author_rank: Super Contributor
timestamp: August 07, 2018, 02:23:39 pm
content: Quote from: In Vacuo Veritas on August 07, 2018, 02:14:49 pmDon't be an asshole.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: August 07, 2018, 03:30:26 pm
content: Quote from: andersm on August 07, 2018, 02:23:39 pmHe's claiming to be an EE and asking sub-hobbyist questions. The table of contents of the PDF is right there for everyone to see. Pressing CTRL-F should not be a PHD-level skill.

author: EEVblog
author_rank: Unknown
timestamp: August 07, 2018, 03:35:58 pm
content: Quote from: In Vacuo Veritas on August 07, 2018, 03:30:26 pmYes, but as mentioned, you don't have to be an arse about it. I've noted a few comments like this from you recently, be careful.

author: In Vacuo Veritas
author_rank: Frequent Contributor
timestamp: August 07, 2018, 04:39:00 pm
content: Quote from: EEVblog on August 07, 2018, 03:35:58 pmNoted.

author: cv007
author_rank: Frequent Contributor
timestamp: August 07, 2018, 05:59:24 pm
content: I bought some of these- PIC16F15354Mouser part# 579-PIC16F15354-I/MLThese were 6x6 0.65 pitch, and they did not have a center pad. I'm now not even sure how I figured out how to even get these as I don't see the ML footprint in the datasheet (although 6x6 is mentioned). It must of made sense to me at some point.QuoteI think it may be meaningless anymore what they show for those pads. I think there were other threads on this- showing the pad in documentation, but there is no pad. My guess is they are dropping the pad on newer parts, but the documentation is lagging.You almost need to order one to find out what you will get. I would guess there is no pad for that part.

author: KL27x
author_rank: Super Contributor
timestamp: August 08, 2018, 11:30:36 pm
content: OP:The center pad on 8 bit PICs does not have to be connected, unless you are running into dissipation/thermal issues. I've heard others using them in professional applications don't always connect it, if hand soldering. I don't do it, either.BUT, you should1. Not worry about bridging, because it's not going to bridge. The center pad is as far away from the outer pins as it needs to be. If there were a problem, they would have made the center pad smaller.2. If you are going to omit the center pad on the PCB (for no reason), you should consider altering the soldermask to prevent soldermask under the chip. Soldermask will raise the chip a little and make the outer pads into a shallow trench. This will apply to any board, but especially to flex boards where the mask is actually a 1 mil thick coverlay. For QFN chips, there is typically no soldermask applied between any of the pads; hence no soldermask under the chip's footprint, at all. Contrary to popular belief, the reason for soldermask is not to prevent bridges. It's there to prevent tinning of things which are not supposed to be tinned and to prevent oxidation/corrosion. Flux and proper pad sizing/spacing is what prevents bridges.So you can omit the pad if you want. But having any soldermask under there can increase your error rate in the reflow and/or hand soldering. So you can't really use that space for routing other traces. I.e., AFAIC, there's no compelling reason to omit the pad, except to give yourself a pat on the back for doing something that wasn't necessary. The only reason I can even imagine is if you are going to paint the pads with a soldering iron and then use hot air reflow... then you wouldn't have to be as careful where you put the iron. But I suggest if hand soldering, using an iron without hot air will be faster than this, anyway.

author: westfw
author_rank: Super Contributor
timestamp: August 09, 2018, 01:35:45 am
content: QuoteMeh. If you're going to do other things that will disallow the use of traces and vias under the chip, then you might as well put the pad there...

author: ocset
author_rank: Super Contributor
timestamp: August 09, 2018, 04:53:43 am
content: QuoteThanks thats interesting, but in gerneral, solder resist is usually placed over the FR4 of the board...and this woudl never be tinned, and would never oxidise?

author: KL27x
author_rank: Super Contributor
timestamp: August 09, 2018, 09:12:51 am
content: ^What reason would there be to cover just a couple mils past the edges of the traces/pours and have a weaker bond to the board and make your silkscreen bumpy and less legible? And why would you need to prevent bridges between traces/pours that are covered? But between the pads of the finest pitch IC's and BGA's, you explicitly do NOT put soldermask?You might say soldermask also seals the pores of the FR-4 from solvents and possibly moisture, too. So maybe there are other reasons. For reflow process, soldermask IS absolutely necessary.... over the traces to keep the solder on the pads. Else the parts just drift around out of alignment and there's not enough paste. Between pads where there isn't any copper/trace, it is not needed.Gullwing pins extend down below the bottom of the IC to clear soldermask. With enough space, a manufacturer with high enough resolution can even put soldermask between the pins. QFN and the L-something-something and BGA pads are flush to the chip. Soldermask anywhere under the chip is ill-advised.