For option 1:
You should connect pin 5 to the top ground plane. Makes the return path for C4 and C5 a lot shorter.
You are correct, I somehow missed that. Thanks. What about the trace from the SS pin? (look at the picture in my upper post, also posted today)
Low speed I/O tracks can be fairly long without problems, provided the the CPU GND and Vcc are going to be clean as they are basically attached to one of these. Now you have good GND connection to a plain and the Vcc is well decoupled, you should be OK.
Great, thank you all. I guess everything below 1 Mhz is considered low frequency?
Not quite, rise and fall time come into it. Series resistors or ferrite beads are used to limit these.
What matters a lot is the position of external connectors. All on one side of the board is good, which is why switched mode power supplies are made like that. Cables on opposite sides is the worst as you are making a dipole antenna driven by any noise across the board.
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You have many unnecessary vias and tracks in your ground plane. C1 is virtually useless. Your local ground plane should have several vias stitching it to the main ground plane. The oscillator layout is improved but there’s room for more still. And that’s just on the part of the board we can see in the pics above.
You could clean the layout up a bit by rotating R1 180 degrees, run the ‘SS’ track outside J1, remove the vias and track under U2. For a start.
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@davidsrsb I understand what you have written. If the mcu produces a 36kHz pwm, that means that the signal contains higher components (the sharper the edge, the higher the frequency components). Am I correct? But that means it depends on the mcu how good the pwm signal will be and what higher frequency components it contains. Am I correct?
@1.21Gigawatts
I have rotated the R1, thanks. Why should “SS” track be outside the J1 and not beneath it? (J1 is a switch). I also cleaned up that unnecessary via and track under U2. Why do you think C1 is useless? Isn’t that a standart practise that you put a decoupling capacitor at every side of MCU where Vcc pin is? Now I have two capacitors, one at each side.
Run the ‘SS’ track around the outside of J1 so you don’t need to jump it with vias and a track in the ground plane.
As for C1, see this post:
Complex ICs with a lot of I/O often have separate power pins on each side to reduce ground bounce. Often the Vcc pins are not even internally connected. One 100nF ceramic on each side with a Vcc pin is good practice. I don’t bother with a Vcc plane until I have 6 or more layers
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@1.21Gigawatts I read it and I understand the difference between decoupling and bypassing. But I still don’t get it what is the reason for not using C1. Is it because of the position of the power supply, which is on the top right, so only a capacitor on the right (C6) can do its job (return path for high frequency signals)? I posted a photo below.
I also changed my traces a little bit, the ground plane is now even cleaner.
C6 is okay, other than being compromised by having to decouple pins 4 & 6 as well. It would be much better to provide a more direct connection to 5V for pins 4 & 6 of U2. You could route a track from the pins 6&7 of U1 up and around D1,D2 to supply pins 4&6 of U2. Ideally both pins should have their own decoupling capacitor. Don’t worry about reducing the size of the local ground plane. Zones don’t have to be rectangular, you can be a little more creative. 
Yes, much better. While it’s not extremely important for this particular board it’s always good practice to reduce unnecessary vias as well as maintaining the integrity of your planes. There is one more ‘jump’ you could eliminate by moving the ‘9V’ track. Also, remove any tracks that make ground connections, such as the track between C2 & C3. Instead use a short track and a via to connect directly to the ground plane. It also wouldn’t hurt to make your 5V tracks wider if you have room.
Thanks for great suggestions. I did all, I just didn’t figure out how to avoid the ‘9V’ track, but it’s a short one, so I think it’s not a big deal… You can see the changes in the photo below. I modified the local plane, but I am not 100% sure if I could reduced it more.
So as I understand you are saying that ideally every power pin of a chip should have their own decoupling capacitor. But in the last post, you said that C1 is useless. I don’t understand 
(I am very interested in this topic and looking forward to your answer). Thanks!
C1 was useless, or at least less effective, before as it had no direct connection to your supply voltage (5V). You have corrected that now. And yes, every power pin should have it’s own decoupling cap if possible.
That 9V track could go down the left side of pin 2 of the LED, but it’s not important. But you should remove the 5V track that crosses under U2.
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Thanks. I moved the 9V track and removed the 5V track under U2.
One more important question before sending it to the manufacturer. (I have asked this before and @Rene_Poschl answered nicely, I just wanna be sure if I can do it with my equipment). There will be no soldermask between the pads of the U2. Is that a problem since I am hand soldering with chisel tip? I solder SMD chips before with the dragging technique (just drag the tip across the pads), but is this possible if there is no soldermask in between? I also have a flux pen.
I hand soldered a similar pcb a few months ago.
I don’t really use the drag method though. (At least not in the way that you put a lot of solder over all pins and wig the excess away afterwards.)
I put very little solder onto the tip and solder each pin separately under a microscope.
But I suppose you have a very fine tip? I don’t have a microscope and only the chisel tip. Should I replace the chip?
This is the tip i use.
http://www.ersa-shop.com/ersa-ersadur-lötspitze-für-itool-gerade-meißelförmig-p-819.html
The microscope helps a lot. But if you have a good magnifying glass it might already be enough. (I use 20x magnification setting on the microscope while soldering. I can set it to 40x magnification but i never used this setting.)
This is the microscope i use. http://en.microscope-online.com/nl-gb/products/microscopes/stereo-microscopes/novex-p-series-2/novex-stereomicroscope-p-20-led/
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I usually just manually spread a little solder paste on the pads, mount the parts, and pop it into the toaster oven. Put an oven thermometer in there with it and manually control the profile. Works every time although I do occasionally get the odd tombstone.
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Solder doesn’t stick much to plain FR4 much more than solder resist, in my experience. I’ve also used drag technique, and the lack of soldermask doesn’t seem to be a problem. Either way I get a few blobs that need to be removed with solder braid. I don’t flood fill with solder though - I don’t think that is a good way.
One thing I try do is ensure that tracks enter the IC pads straight rather than an angle, and also avoid running tracks near the pads. Sometimes the soldermask exposes a bit of a nearby track, which risks shorts. Also if you are doing a board that might be “experimental”, avoid running tracks under components, as they are very difficult to rework.
Here I would move the 5V track away from the GND pad a bit.
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Hand-soldering SMD components is an area where there is more “art” than “science”, and each artisan develops his preferred style, methods, and techniques. When doing an entire board I use solder paste dispensed from a plastic syringe, and an electric skillet (purchased quite inexpensively at a second-hand shop) as my “reflow oven”. I think it’s documented in old posts of mine - try searching with “skillet” as a keyword. There are also several good internet pages on the topic.
For rework, repair, and touch-up after reflow I use a fine conical tip in my soldering iron. The toolmaker’s microscope (similar to the one already mentioned) is definitely helpful. I think the one I use cost my employer about US$200 and has 10X and 20X magnification; the lower magnification is quite adequate. Having said that, unless I have a large volume of SMT work at hand, I usually get by with just a decent-quality desk-mounted magnifying light.
When adding SMT components to an assembled board, the combination of soldering iron (with the fine conical tip) and solder paste works well for me. My attempts at using the “drag technique” have never been satisfying to me. Perhaps that would change with extensive practice. My tray of soldering tools includes the finest gauge, rosin-core, wire solder I can obtain; some wooden toothpicks; stainless dental probes; cotton swabs; pure isopropyl alcohol (not the 70% stuff from the drugstore); and fine-braid solder wick.
Dale
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