Can you please elaborate a more by what you mean as resolution that is available in case of green PCB and not black PCB ?
I wish I knew. PCB fab vendors can be remarkably cagey when it comes to process details, especially when having to admit something isn't up to spec.
I basically want to know the difference that will be observed between the Green and Black PCB , Is it the visiblity of tracks or the layout or something related to electronics.
That too is a good question. I know a bit more on this, but again, this involves some interpretation from what a PCB assembly vendor is willing to admit. (Two different vendors here, the "PCB fab" vendor actually makes the circuit board, the "PCB assembly" vendor solders all the parts onto the board)
The solder mask is a non-conductive film applied over the entire PCB, except the pads where you're supposed to solder parts. Its main purpose is to prevent the solder from flowing down the traces, away from the pad. A secondary purpose is providing a very low surface adhesion to the molten solder, so if any solder does accidentally get incorrectly applied off the pads, it will tend to be pulled towards the pad. The solder adheres to the plated metal surface on the pad, and has very little "stick" to the surface with solder mask.
With the black solder mask, we were seeing defects in the mask material. Again, I don't have a good feel for the numbers (going back to the difficulty of getting vendors to be forthcoming with quality problems that expose problems in their process), but it's probably something like 1% to 5% of the boards had these issues, in varying degree. I also don't have really good info about what the defects really were, but some I saw appears to be missing solder mask, or perhaps really thin spots in random places. It's remarkably hard to know after the board is built.
The board can still be soldered and work perfectly, even without the solder mask.
By far the worst problem we saw was difficulty inspecting the boards. Our assembly vendor uses a camera system. I haven't personally used this system or even seen it in operation (they did offer to give me a demo, but the timing hasn't worked out... I'm extremely busy and I tend to work odd hours). So this is only based on what I've been told, so you're hearing it 3rd hand. Apparently the camera views the PCB from multiple angles and image recognition software analyzes the solder joints. The problem is only an ideally shaped solder joint passes. A perfectly fine solder joint that has the solder distributed differently, because some of it "escaped" onto the trace underneath (especially to the large center pad on the QFN chip) or down the trace away from the part, will "fail" on the camera inspection system. Then a real human needs to inspect the board.
The vast majority of the cost on Teensy comes from 2 places: the silicon and the PCB assembly (software development isn't counted in the cost per board). If anyone ever wonders how Sparkfun is so successful, consider they do all their own assembly work and they do very little software themselves! Their main cost is the silicon, and they can afford to pour a lot of money into website development, which clearly shows in the awesomeness of their site. PJRC operates on a very different, not nearly as profitable model, where most of my focus is on developing software. Teensy3 is a 4-layer board, which adds cost too, but not on the scale of the chip and assembly. Much of the reason we can make Teensy affordable is because I work pretty closely with a couple local PCB assembly vendors. These troubles with the solder mask were mostly about automated inspection.
In extreme cases, solder mask problems can lead to unconnected pins or even shorts. If the solder that was supposed to connect the pin all flows away because the connecting trace is exposed, it's possible to end up with an unconnected pin. But that's pretty rare. The pad and the pin are plated with metals that optimize solder adhesion, and the solder is already touching them. For an open to happen, this needs to coincide with some other problem, like the part wasn't placed accurately or the PCB wasn't perfectly flat, which just don't tend to happen. It's also possible to end up with shorts between the pins. But again, for that to happen there needs to be solder deposited between the pins, which ideally shouldn't happen if the stencil is correctly aligned when the solder paste is applied.
If any part of this sounds well researched, let me assure you that is only in hindsight. The reality of dealing with these manufacturability issue is far less certain. We would get a batch of boards in, or a partial batch if we were close to running out (even though they would run all the panels on the pick-and-place and reflow machine at once, they were struggling with inspection which limited the rate we'd get them), but often we'd test and package them as needed to ship orders. Soldering the pins to boards takes a lot of time, which is why we charge $3 more for the pins soldered, and it puts a limit on how quickly we can test and package a batch of incoming boards. That limits how fast we can get feedback to the PCB assembly vendor. The pain they suffered having to visually inspect so many boards is largely forgotten and turns into a sign of relief when we report that virtually all of them actually passed electrical testing here. Remember, they're not not always very forthcoming about limitations on their end, with much of the communication issues internally between their production people and their management, so inspection issues didn't become known to me until recently. On this side, Robin, Erin and I are highly focused on delivering good products, shipping orders fast and a huge number of day-to-day things that come with running a tiny 3-person company. Optimizing a production process, where only a tiny percentage of boards actually fail, well, just isn't as high a priority as a mountain of other stuff. Until mid-2013, to be honest, a lot of Arduino libraries still needed porting for Teensy 3.0, so I just wasn't able to focus on much else that wasn't extremely urgent. The easy answer is to just write off and discard the small number of bad boards. It's taken about 1 year and especially the last several months of working closely with our assembly vendor to really refine this process. On both sides, we've really increased our attention to optimizing the manufacturing process. The same happened years ago with Teensy 2.0 and Teensy++ 2.0. Most of this stuff is very mundane details you'll never see. Another upcoming change is a very small adjustment of the size of the center pad under the Mini54 chip, and perhaps to the 4 smaller cutouts in the stencil that put 70% solder paste coverage on that big pad. Usually these refinements are invisible details. Much to my surprise, one of the problems turned out to be black solder mask. The change to green is pretty easy to spot.
