LED Matrix PCB
Jay Kickliter
Posts: 446
I'm attaching several files of a design I'm almost ready to send to Sunstone. If you have a spare minute, please let me know if you see any errors.
It's a little badge with an 8x8 LED matrix. ~1.4x2 inches. Driven by a Propeller, and has onboard Li-Poly charging and USB programming. On the back you can solder either a CR2450 coin-cell holder (for the rechargeable Lithium cell from SparkFun), or a JST connector for a leaded single-cell Lithium.
I did my best to lay out the parts in a logical manner, but size and package constraints made for some difficult routing. Obviously, by looking at it it was autorouted by freerouting.net's autorouter, but I did a lot of clean up, except for the LED's, which is why that routing looks weird.
I'm including the Eagle source files also, so if anyone is interested you can do whatever you want with it. Also there is a .dsn file, which is what I fed into the autorouter.
thanks,
Jay
Post Edited (Jay Kickliter) : 12/3/2009 3:50:52 PM GMT
It's a little badge with an 8x8 LED matrix. ~1.4x2 inches. Driven by a Propeller, and has onboard Li-Poly charging and USB programming. On the back you can solder either a CR2450 coin-cell holder (for the rechargeable Lithium cell from SparkFun), or a JST connector for a leaded single-cell Lithium.
I did my best to lay out the parts in a logical manner, but size and package constraints made for some difficult routing. Obviously, by looking at it it was autorouted by freerouting.net's autorouter, but I did a lot of clean up, except for the LED's, which is why that routing looks weird.
I'm including the Eagle source files also, so if anyone is interested you can do whatever you want with it. Also there is a .dsn file, which is what I fed into the autorouter.
thanks,
Jay
Post Edited (Jay Kickliter) : 12/3/2009 3:50:52 PM GMT
Comments
The only possible problem I see is the center pad for the QFN. I would reduce the center pad to prevent possible shorting if assembled by automated equipment. Also you do not want that much solder paste in the center, it will cause the chip to float on it....and possibly not connect the outer pads.
I see you have made provisions for hand assembly. Good thinking.
James L
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James L
Partner/Designer
Lil Brother SMT Assembly Services
Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!
I read that rounded SMT pads reflow better, any opinions?
How could you tell I intended hand assembly?
thanks a lot,
Jay
You use a single pad for the centre, the squares are for the stencil.
Leon
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Amateur radio callsign: G1HSM
Jay,
I always make the center pad smaller to prevent problems. The manufacturer recommendations are for bare copper pcbs, which are going to be assembled by state of the art equipment. It is much easier to just prevent problems. I typically reduce the total size by 30%. Many will claim this will cause problems, but I have yet to encounter a problem by doing it. Even when reducing the solder amount, I still reduce the center pad size. The less to go wrong the better. I like having the 30% of added security of no shorts to ground, even though the pins still could be shorted to each other. Some times I do not reduce the pad, but reduce the soldermask opening. This does the same thing without reducing the heat sink effect of the center pad.
Yes, rounded pads do reflow better. This is due to molten solder working like a drop of water, it doesn't do corners well (because it is round). I don't find that it's really important. If you get the pad soldered, the little corners which are somewhat dry present no problem.
I noticed the via under the QFN. Only hand assemblers put one via under a QFN. If it were for heat dissipation there would be more than one.
James L
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James L
Partner/Designer
Lil Brother SMT Assembly Services
Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!
Post Edited (James Long) : 12/4/2009 6:41:03 PM GMT
I don't see any bypass caps near the Prop or EEPROM. There probably isn't room topside to add them at this point; but since you'll be doing hand assembly, maybe you could put them on the bottom. For the Prop, a couple 0.22uF 0603 cer. caps very near Vdd and Vss on opposite sides of the chip will do. For the EEPROM, one 0.1uF 0603 cer. cap near Vdd is fine. The EEPROM cap could possibly eliminated if one of the Prop Caps is nearby. I realize you're running at slow speed from battery power, but you're also driving those LEDs, which will cause current transients, so better safe than sorry.
-Phil
Post Edited (Phil Pilgrim (PhiPi)) : 12/4/2009 6:45:38 PM GMT
The only thing I'm not sure about is if it is a bad idea to let the VDD polygon creep under the Prop like that.
-Phil
I agree with Phil. If you ever decided to have those assembled by an automated process, I recommend at least a 0.010 spacing for those. That group looks to be a little too close. But it is hard to tell by the picture.
James L
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James L
Partner/Designer
Lil Brother SMT Assembly Services
Are you addicted to technology or Micro-controllers..... then checkout the forums at Savage Circuits. Learn to build your own Gizmos!