Need some FPC Advice! - Page 1
Forum section: electronics / beginners

author: LightningBug
author_rank: Newbie
timestamp: November 05, 2013, 05:18:48 am
content: Hi all, long time viewer of the blog but new to the forumI'm a noob to electronics design but feel i have come a long way in the last 3 months. I am just finishing my first design for a magnetic encoder and could really do with some advice from more experienced people. The killer here is the incredibly small envelope i have to work within and the only solution i have is a custom FPC that, due to a limited hobby budget, is gunna have to be a home-brew.I believe the fabrication of the flex strip is not going to cause me too much of a problem. It will be a piece of single sided 2oz Pyralux, with the etch mask directly printed on to it by a solid-state Xerox ColorQube.I need to mount an AMS AS5050 encoder IC on to the end of a flex strip that is no more than 4.6-4.8mm wide and the IC is a 4mm 16-QFN 0.65mm package.... not much room for a standard landing pad or traces!!So... as much info as i can give is what i guess is needed now... and so you dont have to look up the specs:Pinout from the topProposed FPC:So basically i redesigned the landing pad to try and use the pads themselves and the "Not Connected" pins as part of the circuit. I also had to try and factor in minimum trace width and spacing for home-brew PCBs:mm milsA 2.95 116A2 2.95 116B 1.50 59B2 2.25 89C 0.30 12C2 2.25 89D 0.35 14D2 0.35 14E 0.65 26Hopefully you all have an idea of what im trying to do, i can now get to the questions...1. Is this going to work in practice?2. Is the lob-sided and non-standard landing pad going to cause a problem when mounting the IC? Will the surface tension of the solder want to pull the IC upwards and off center?3. Just how the hell do i solder this thing anyway :/ I only have access to a basic solder station and a "paint stripper" style hot air gun. Im guessing i should be sending this out to be done by someone else!thanks in advance for any help or advice you can give me, on getting this done!

author: SeanB
author_rank: Super Contributor
timestamp: November 05, 2013, 06:29:15 pm
content: I would suggest that the pins labelled "Not connect" should be left as not connected. While they may be unused in your circuit, they probably are connected internally in the chip to some points and connecting them to a trace or to a power rail may cause the chip not to work.

author: LightningBug
author_rank: Newbie
timestamp: November 05, 2013, 07:27:02 pm
content: Thanks for the input Sean:)I did get one of these ICs mounted up on a 16 pin DIP adapter board and from all my testing, it appears that the Not Connected pins really are "Not Connected" to anything inside the Chip....Well at least it all works as i want it to with the extra pins connect to ground and VCC.I'm really after some feed back on building my own flex strip and then how to solder the chip to it.Thanks again

author: wraper
author_rank: Unknown
timestamp: November 05, 2013, 07:55:01 pm
content: Quote from: LightningBug on November 05, 2013, 07:27:02 pmWhile it seem to work and look like not connected, those pins may be used in production for testing. So this can cause all sorts of hidden problems. Starting from increased power consumption to more funny things.

author: LightningBug
author_rank: Newbie
timestamp: November 05, 2013, 09:10:54 pm
content: Ok... i get what your saying, i didnt think about that. So i guess i need to flick AMS support an email and find out for sure or its back to the drawing board to see if i can source some cheap double sided Pryralux.Anyone else have ideas on soldering the final thing? from the little i know, its gunna be reflow with solder paste and just rely on the kapton to act as a solder mask.thanks again!

author: LightningBug
author_rank: Newbie
timestamp: January 19, 2014, 11:28:11 pm
content: Just an update on this one, it all worked out fine in the end. The soldering was a little unconventional, i ended up taping the FPC to a steel ruler and heating it from below, just to get enough of a thermal mass going!Its just one part of a 30 year old puzzle that I'm trying to solve…. more info on the project can be found here…http://keypicking.com/viewtopic.php?f=102&t=8763thanks, ill be back with more questions again!