Wednesday, August 3, 2011

More on The Spiral Challenge

A Homebrew_PCB Yahoo Group User; DJ, suggested that I try to fix my flawed 10 mil Spirals as reported on my previous post. With the aid of a 15 power Microscope I was able to fix each of the "opens". A few flaws were hard to find, but tracing them around-and-around with an Ohm Meter under the Microscope did the trick.

Fixing the trace breaks was a job for very sharp soldering iron and a steady hand. Building a solder bridge at this scale is an interesting process.

The Flaw Fixed with a Solder Bridge
Click Image to Enlarge
The above photo show a typical solder bridge fix. The Tee Pin is included as a pointer and for scale.

Solder Bridge

When all flaws were fixed, each of the trace resistance was measured end-to-end and then between traces:

  • First Trace = 23.4 ohms
  • Second Trace = 23.3 ohms
  • Trace to Trace = multi-meg ohms

The Challenge suggests the resistance should be 18.0 ohms per trace.

While investigating the Double Spiral Challenge, it seemed like my traces were smaller than proposed by the Challenge. But, I did not have anything to verify my PCB Toner Transfer Method fabrication results.

Until, I thought of including a calibrated scale inside of the photos. But alas, I do not have a small calibrated scale, so I decided to use a coin, but it was far to big. Then I hit upon the idea of using an Standard SMD part. The following photo contains a Standard 0805 SMD Resistor, from it a scale can be calibrated and distance measured.

Looking at the circuit board at this Magnified scale you wonder how any electronic device could possible work, with all of its flaws.

I may need to think about this more (later, it is very late at night) and now my thinking is dull. An update will follow.

The Tee Pin, Solder Bridge and the 0805 SMD Resistor
A Side Note: While futzing with my Cell Phone Camera and the add-on Attachment Lens (which was used to take these photos), I inadvertently twisted the Lens into two parts. With only the Lens base part attached, the Lens becomes a much higher power Macro Lens.    Hum, . . I wonder why I did not know that before.


I received the following email from DJ, the owner/keeper of the Challenge, via the Homebrew_PCB Yahoo Group, it is included here for completeness.


    Here's a tip: you know the track-to-track spacing is 20 mil (you did say 10/10 pattern, yes?). Given that, you can calculate the actual DPI of the photo (about 4300 dpi for the spot I was measuring). Also measure a track, and you can compute the actual track width (about 9.2 mil in some spots, 7.1 in others).

    But simply measuring pixels-of-copper vs pixels-of-space should tell you how close to "perfect" you are.

    Your ohms are a little high, some things that might cause that:

    • pinholes cause "necks" in the copper which add resistance
    • cleaning/scraping/sanding the copper might thin it somewhat
    • thinner traces on average would increase it

    If you just go by the ohms ratio, it says your average trace width is 18/23 * 10 = 7.8 mil, which sounds close enough.

    Did you measure the high ohms between the two tracks?

    Also, if you have a flatbed scanner, they're often useful for high-accuracy PCB scans.


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