My first SMT design arrived today (and my second PCB overall) - Page 1
Forum section: electronics / projects

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 08, 2015, 12:43:57 pm
content: I'm excited! Hope I'm posting in the correct section of the forum.Here's my first SMT design in the physical world. I welcome it with much joy. Produced through Seeedstudio (ps: the white solder mask was not my choice):Goofed up one footprint to be 0402 when I ordered 0603. DOH. I also regret not going with a through hole USB connector, but these things will have one user; that's me. They'll also have a very short life, about two months. So all in all I'm happy (at least if I get the damned things built and working).I've also got an STM Max reflow oven on it's way. It will be my first time assembling using this technique.It's an "IoT" playground with an STM32F1, CC420 and room for the ESP8266.Wish me luck guys

author: wraper
author_rank: Unknown
timestamp: April 08, 2015, 12:57:36 pm
content: Why most of the SMT pads are rounded?

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 08, 2015, 01:27:00 pm
content: I wondered about that myself, I use a footprint library for the house that will eventually do the production. Maybe it's to accommodate with some need they have for preventing tombstoning?I have no idea, I was just told to use them.

author: zapta
author_rank: Super Contributor
timestamp: April 08, 2015, 03:53:46 pm
content: Look very nice.Do you plan to use a stencil?You can try gluing the big connector somehow such that it doesn't peel the pads when you apply force.When you place it in the oven raise it a little bit with small pieces of PCB to get more even temperature (I learnt it from one of Ian's videos).

author: rx8pilot
author_rank: Super Contributor
timestamp: April 08, 2015, 04:14:08 pm
content: Quote from: wraper on April 08, 2015, 12:57:36 pmI think it helps with paste release when using a stencil. Could be wrong.....

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 08, 2015, 06:29:08 pm
content: Thanks:)I did order a framed stainless steel stencil. One of the most beautiful things I've ever received in the mailI won't do the bottom with reflow, I'll just solder them manually. Should I still raise the board even if it is single sided? Can you provide a link to the video you speak of?

author: Tabs
author_rank: Regular Contributor
timestamp: April 08, 2015, 06:37:36 pm
content: Hi,In general, its difficult to achieve a 90 degree corner when creating structures with an etching process. This applies to the PCB board and its stencil (if using etched stencils or lazer cut ones). The corners are always rounded to some degree. The effect is less noticeable on large pads.Part of the problem with a rectangular pad is that the rectangular hole in the stencil makes it difficult for the solder paste to be pushed into the corners of the pad. If you've ever used a stencil and manually applied the paste (eg on a prototype board from PCB Pool or similar) then sometimes you can see 'air pockets' or gaps in the paste coverage of some pads. These gaps are usually in the corners. This sometimes happens in an automated pasting machine.The result is less paste on the pad and when this flows it usually flows into the corner where the surface tension is stronger. This pulls more solder away from the centre and you can see the result as less than optimal side fillets on the final joint.Lead free solder takes a higher temperature to flow and when it does its never as 'flowy' as the leaded stuff. Not sure if this is caused by flux boiling off at normal temperatures leaving a much dryer solder alloy when it flows. The result is more solder at the centre and this causes less than optimal heel/toe fillets.Rounded pads are used to reduce this problem. They are usually contained within the perimeter of the rectangular pads. This results in a smaller pad which reduces the amount of control and precision required in the etching process, but also reduces the paste on the pad. There isn't a large amount of information on the real performance difference between the two since the IPC have no documentation on rounded pads and because most manufacturer recommended footprints use rectangular pads.Most companies who have looked into the problem usually do this because their internal quality in SM assembly could be improved. The designers have optimised rounded pad sizes in collaboration with their assembly house. I'm not sure how much of the increased performance was down to having rounded pads and how much was down to the fact they were optimised by trial and error. eg, you may have got the same performance by optimising the rectangular pads. In fact IPC recommends that each manufacturer optimise the footprint dimensions (generated by the IPC formulae) to their own manufacturing process.

author: tszaboo
author_rank: Super Contributor
timestamp: April 08, 2015, 06:43:21 pm
content: IDK where this negative silkscreencame from, but I really hope people stop using it one point.

author: langwadt
author_rank: Super Contributor
timestamp: April 08, 2015, 07:01:57 pm
content: Quote from: NANDBlog on April 08, 2015, 06:43:21 pmafaict it is white solder mask with black silk screen, nothing negative just uncommon colors

author: zapta
author_rank: Super Contributor
timestamp: April 08, 2015, 07:40:35 pm
content: Quote from: alexanderbrevig on April 08, 2015, 06:29:08 pmLook here around 2:50. He has a few more videos on the subject.I am using a similar one and am very happy with it.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 08, 2015, 07:42:11 pm
content: Quote from: NANDBlog on April 08, 2015, 06:43:21 pmQuote from: alexanderbrevig on April 08, 2015, 12:43:57 pmI'm not really a fan of it either. I kind of think it looks clean, but I don't like to work with it. Hard to follow traces through the silkscreen (though as I've printed out the gerber there should be no troubles for me).

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 08, 2015, 07:42:53 pm
content: Quote from: zapta on April 08, 2015, 07:40:35 pmThanks! Watching it now

author: tszaboo
author_rank: Super Contributor
timestamp: April 08, 2015, 08:20:33 pm
content: Quote from: alexanderbrevig on April 08, 2015, 07:42:11 pmOh, that is soldermask and not Arduino white sprayed everywhere "artwork". That is OK. I guess it was panelised together with a bunch of LED boards.I hope you are not planning to run the thing from the coin cell. They dont really like supplying 100s of mAs of current.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 08, 2015, 09:02:56 pm
content: Quote from: NANDBlog on April 08, 2015, 08:20:33 pmWe specifically ordered white, but it was not my decision.Quote from: NANDBlog on April 08, 2015, 08:20:33 pmThe ESP8266 will only be used for a version that is powered from a mains power (JST plug)For the leaf nodes I hope to get at least some lifetime using the coin cell, but the JST that will be mounted on there could also be used instead if coin cell proves too little juice (I suspect it will, time will tell).

author: rx8pilot
author_rank: Super Contributor
timestamp: April 09, 2015, 01:12:43 am
content: Are you able to see the traces through the soldermask or is it opaque?

author: Falcon69
author_rank: Super Contributor
timestamp: April 09, 2015, 03:22:56 am
content: Quote from: zapta on April 08, 2015, 03:53:46 pmGood Idea! I never thought of that. I sometimes have problems with components sitting directly above the wire of the rack inside the oven.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 09, 2015, 09:17:21 am
content: Quote from: rx8pilot on April 09, 2015, 01:12:43 amI can see them vaguely at an angle (you see the tendency in the photo f.ex. the trace to the antenna connector). It's not great but it's OK for now.For interest, what's your preference on solder mask - and why?

author: rx8pilot
author_rank: Super Contributor
timestamp: April 09, 2015, 05:02:39 pm
content: In general, I don't care much about the soldermask as long as it does the job of masking. I like being able to see the traces during prototyping and repair so I don't have to rely so heavy on computer and paper references.The vast majority of PCB's are embedded in an enclosure and never see the light of day.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 16, 2015, 02:03:14 pm
content: Here's my first pass through the reflow oven:Quite happy about the result actually, those solder bridges should be fixable with some solder wick.I hope it's not going to turn out as one fancy magic smoke generator though

author: zapta
author_rank: Super Contributor
timestamp: April 16, 2015, 02:40:36 pm
content: Quote from: alexanderbrevig on April 16, 2015, 02:03:14 pmThe black on white looks very nice. I will try it with one of my next PCB orders.You can also fix bridges with flux (I prefer tacky gel flux) and dragging a clean tip. This will also leave the correct amount of solder on the pads.

author: Tabs
author_rank: Regular Contributor
timestamp: April 16, 2015, 09:34:15 pm
content: Quote from: alexanderbrevig on April 16, 2015, 02:03:14 pmLooks good.Did you use tented vias? I cant tell because of the white resist. If not, try it next time. The boards will look much better.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 16, 2015, 10:14:27 pm
content: Quote from: zapta on April 16, 2015, 02:40:36 pmThe black and white grows on me too.Thanks for the tip! I'll definitely try that out.Quote from: Tabs on April 16, 2015, 09:34:15 pmThanks. I used tented vias for most of the stitching vias between GND fills on top and bottom. For some reason I didn't tent them all (I was not sure if it would be a problem with too wide radius and such, I'll try to tent everything the next time around. I completely agree the parts of the boards with tenting is prettier than the vias one can see.

author: Tabs
author_rank: Regular Contributor
timestamp: April 17, 2015, 01:20:05 pm
content: Quote from: Tabs on April 16, 2015, 09:34:15 pmThanks. I used tented vias for most of the stitching vias between GND fills on top and bottom. For some reason I didn't tent them all (I was not sure if it would be a problem with too wide radius and such, I'll try to tent everything the next time around. I completely agree the parts of the boards with tenting is prettier than the vias one can see.I think 0.8mm is largest tented size that is standard and 1mm is considered special. These are the capabilities of our high volume pcb providers, It depends on the pcb fabricator.

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 20, 2015, 02:52:51 pm
content: I NEED TO RANT!STM for some reason (please tell me this is not standard) decided to put two circles that I'm used to meaning pin 1 on the chip.I did thesillymistake of trusting my intuition so I thought the larger one had to be pin 1.Turns out, it's the smaller one, so when you read the text level it's the pin to the bottom-left below the text...

author: that_guy
author_rank: Contributor
timestamp: April 20, 2015, 04:03:09 pm
content: Quote from: alexanderbrevig on April 20, 2015, 02:52:51 pmI definitely sympathize with you. Unfortunately that larger recess is a rather common trap for the unwary - a mistake I made myself with a Xilinx FPGA some years ago..QuoteThe bottom-left location of the recess relative to the text does seem to be a reliable unwritten standard.

author: zapta
author_rank: Super Contributor
timestamp: April 20, 2015, 04:27:56 pm
content: Quote from: that_guy on April 20, 2015, 04:03:09 pmST only or all manufacturers?

author: alexanderbrevig
author_rank: Frequent Contributor
timestamp: April 20, 2015, 04:34:46 pm
content: Thanks.. I will definitly double check the next time. At least I learned something

author: c4757p
author_rank: Super Contributor
timestamp: April 20, 2015, 04:54:23 pm
content: Quote from: alexanderbrevig on April 20, 2015, 02:52:51 pmYup, it's standard. I know this well andstillrecently fried a microcontroller this way - just wasn't paying attention.It's always the little one.I wonder why they do that. Anybody know what purpose the other mark serves?

author: SeanB
author_rank: Super Contributor
timestamp: April 20, 2015, 05:16:40 pm
content: Ejection pin from the moulding process, used to push out the chip after the heat and pressure has cured the epoxy housing.

author: that_guy
author_rank: Contributor
timestamp: April 20, 2015, 07:52:33 pm
content: Quote from: zapta on April 20, 2015, 04:27:56 pmST MCUs. It's not consistent for other manufacturers unfortunately. For example I've got a Samsung NAND flash TSSOP on the desk here with the ID dimple up the top-left relative to the text. Same with a couple of QFP Atmels.