Prop PCB question
w4fej
Posts: 264
I'm working on a PC board layout using the QFP version of the Prop to save space on the board. I have the pads down and aligned top and bottom. Never having used surface mount stuff before, only through hole, how do I connect the top and bottom pads?? Whats the time honored way to do this?? Add a "via" to each pad??
Thanks, Mike B.
Thanks, Mike B.
Comments
Unlike through-hole pads you only have them on one side, there are no top and bottom pads or need to connect them. What PCB software are you using that is causing you this problem?
EDIT:
That said, if you need to connect to the other side then avoid placing your via in the pad as this causes problems with solder wicking into the via. There are expensive plugging techniques etc but avoid this although if you hand solder it won't make much difference I guess.
In the case of the QFN (or any surface mount component), you only require pads on one side of the board so you could remove that second pad-footprint from the bottom of the pcb and forget about needing to connect the pads !
Does that make life easier ?
-- That all said, you may need to switch some traces to the bottom of the pcb for routing purposes, and in those cases then add the requisite number of vias !
Hope this helps.
1) use 4 layer board with a stackup of :
----a. Layer 1 top, signal, component side
----b. Layer 2 - ground plane
----c. Layer 3 - power plane
----d. Layer 4 - bottom, solder side, signal
2) SMT padstack: L1 SM pad AND nothing on L2, L3, L4
3) via pads: L1 & L4: 22 30 mil pads: L3, L4 = L1 diam. + 10 mil
----a. via hole = pad diam 10 mil (drill 12 mil = 22 10)
4 layer boards cost a little more than 2 layer but are less prone to layout problems, especially when MHz signals are concerned. A 4 layer board is quieter with better isolation for digital, insures even distribution of power and ground, simplifies bypassing and noise reduction, moves heat away from hot parts quicker, and can make for a tighter design.
There should be some tutorials for your layout package. Every minute you spend learning from a tutorial will save you 10 minutes trying to figure it out on your own.
In answer to your question SM pads require vias a short distance away from the SM pad. This is called FANOUT and can be done automatically, power pins first then signals. In general bypass caps are placed early to keep line length short and in such a way as to not block signal lines that will come next. Fanout vias can be placed on the inside or outside of the component pads but power and ground generally go to the outside.
Keep us posted on your progress.
I'm using PCB Express and it's not really a problem, I've just never dealt with SM stuff. I like traces vertical on one side and horizontal on the other hence the question re: the QFP. I now have the answer to my question and will make adjustments as needed....
Thanks for all the responses guys. Love this forum...
Mike B.
If you are using ExpressPCB, I have an example of a QFP Propeller board that you can download the design files for here:
http://www.rayslogic.com/propeller/Products/PPE/PPE.htm
Hi Rayman:
Sorry to be so late in replying to your post but I have been out of town. Thanks for the board files, it answers a lot of questions. I SORT OF had the idea to add extensions to the QFP pads to get the lines on the other sides and it looks like what you have done on a few of the lines.
Thanks for the help..
Mike B.