First PCB - Page 1
Forum section: electronics / beginners

author: jackjr
author_rank: Newbie
timestamp: August 22, 2012, 07:42:51 am
content: Hi,First post here, and first PCB!It's basically a breakout board for the SIM900 GSM module.Since I'm a total beginner, I would like some feedbacks before I send it out for manufacture.The schematics and PCB are attached below.I've also a couple of questions:a) For the RF antenna output, the SIM900 hardware design guide recommends using 50ohms impedance controlled traces, and an antenna matching circuit. This seems to add quite a bit of complexity, so… Is this really needed?In my circuit I just connected directly the RF_ANT output of the SIM900 to the SMA connector, using a short (~8mm length) 25mil trace. Will it work?b) Is the grounding good enough?c) Tenting or not tenting the vias? In my design most are tented.Thanks for reading

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 22, 2012, 08:15:48 am
content: QuoteTherefor, you could simply adding a 47R resistor instead of a controlled impedance trace.QuoteYes, it looks pretty good. Especially the amount of vias.But you should change your VBAT-trace. Because there is an acute angle (close to the yellow square) which should be avoided in any layout on any trace.

author: poorchava
author_rank: Super Contributor
timestamp: August 22, 2012, 08:47:46 am
content: I would consider pouring GND on both layers and then stitching them genrously with a lot of vias.

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 22, 2012, 09:01:45 am
content: I'm not familiar with Altium (by the looks of it), but I thought there is a GND-plane on the opposite layer, which is just not shownWhy should he have that plenty vias on the GND-plane else for...BTW: On Eagle, you got the same problem. There you have to click onRastnest, to make the GND (or whatever net it is connected trough) polygon visible.

author: mtkaalund
author_rank: Contributor
timestamp: August 22, 2012, 09:21:13 am
content: Does this not mean there is missing a connection?if it does then there a quite a lot missingQuotemaybe but I would not count on it, you'll going to need to know the "high" of the copper layer on the PCB in order to calculate it properly. Take a look athttp://smartdata.usbid.com/datasheets/usbid/2001/2001-q1/slua236.pdf

author: PSR B1257
author_rank: Frequent Contributor
timestamp: August 22, 2012, 09:39:35 am
content: QuoteWell, that's the questionQuoteIf you take ausual35µm (which is equal to 1oz) you get about 6.2m? - that's literally nothing.

author: Short Circuit
author_rank: Frequent Contributor
timestamp: August 22, 2012, 09:47:22 am
content: Impedance matched traces on a double sided board are almost imposible, if I recall correctly you'd need traces about 3mm wide. Obviously that does not go well with the GSM module, nor with the connector... For a short link like this one, just use a trace as thick as possible, and you'll be fine.An antenna matching circuit is only needed when the antenna is not a standard 50 ohms model, such as PCB antennas.

author: jackjr
author_rank: Newbie
timestamp: August 22, 2012, 09:47:55 am
content: Quote from: mtkaalund on August 22, 2012, 09:21:13 amThe missing connections are connected to the GND pours.And yes there is a GND pour on each side.I tried to setup a rule in Altium to control the impedance of the trace to 50ohm, which would then automatically adjust the size of the trace, but that did nothing.

author: Thor-Arne
author_rank: Unknown
timestamp: August 22, 2012, 10:19:06 pm
content: Quote from: jackjr on August 22, 2012, 09:47:55 amI suspect it shows as unconnected nets because the GND pour is not there on that capture.The second image don't have those, so I suppose it's ok.Thor-Arne

author: mtkaalund
author_rank: Contributor
timestamp: August 23, 2012, 07:58:54 am
content: Quote from: Thor-Arne on August 22, 2012, 10:19:06 pmYeah for the GND pour, but there is still one connection missingQuote from: PSR B1257 on August 22, 2012, 09:39:35 amYou are right, this can be overlooked/ignored in this design.

author: AndyC_772
author_rank: Super Contributor
timestamp: August 23, 2012, 08:09:08 am
content: I don't see a SIM chip anywhere. Would it not be useful to include the SIM connector on this board too?

author: Thor-Arne
author_rank: Unknown
timestamp: August 23, 2012, 08:42:39 am
content: Quote from: mtkaalund on August 23, 2012, 07:58:54 amYou are right, there's a unconnected net on R4 also.@jackjr:I'm not familiar with Altium (if that's what you're using), but there should be a function to check your design for errors.

author: jackjr
author_rank: Newbie
timestamp: August 23, 2012, 12:16:41 pm
content: Thanks for the replies.I didn't see the missing connections. I fixed them.@AncyC: we can't see it very well, but there is a SIM card connector on the bottom layer.

author: papo
author_rank: Regular Contributor
timestamp: August 23, 2012, 08:44:53 pm
content: HelloYou may want to check the distance of your mounting holes to the PCB edge. On my first PCB, I put these holes too close to the corners of the PCB and the corners just broke off because there was not enough material between the edge and the hole.RegardsMatt

author: atbreaux
author_rank: Newbie
timestamp: September 18, 2012, 07:48:51 pm
content: Quote from: mtkaalund on August 23, 2012, 07:58:54 ami would be careful to ignore these.. because sometimes your pour will not be be connected to the ground leaving a big island pour between traces making the pin connected to that island effectivly not grounded.. i normally just use the odb++ drc for unconnected nets when something like this arises just to be safe.

author: free_electron
author_rank: Super Contributor
timestamp: September 18, 2012, 08:29:31 pm
content: If altium shows you an error or warning it is an error or warning and you NEED to fix it.when pouring polygons always check the option to remove unconnected copper. you will immediately see what is wrong then.

author: Electronic_Project
author_rank: Newbie
timestamp: November 23, 2012, 03:13:36 pm
content: Quote from: jackjr on August 22, 2012, 07:42:51 amI think your design is OK,But I prefer to connect the antenna with a straight short track to moduleand connect the adjacent GND pins of this pin directly to antenna connector GND pins.In addition if you are going to order this PCB without the overlay layer,keep copper out of area marked with a solid fill rectangle.

author: chklopper
author_rank: Newbie
timestamp: June 25, 2013, 09:25:57 am
content: Hii just wanted to know did the break out board work as planned as there is no mention of it acctually working ok.Regards Carel