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.

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UPDATE

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.

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Sweet!

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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The Spiral Challenge

I have been making a lot of Homebrew PCB lately, using the Toner Transfer Method. Must of the boards are prototypes, which will be later be fabricated by one of the online PCB suppliers. I have been feeling very good about the Homebrew Process and have created several small boards with high resolution for very dense SMD projects. The highest resolution projects are 6 mil traces with 10 mil separations between traces or ground plane. My more typical project are created with 15 mil traces and 12 mil separations. Some of the details of these projects have been posted in my previous blogs (note: the selection will include a copy of this post).

The Spiral Challenge

Several years ago, when I first started Homebrewing PCB via the Toner Transfer Method, my resolutions and yield - was Horrible!

I saw the Homebrew_PCBs Yahoo Group Double Spiral Challenge web page, that provided the art work for process challenge, with several standard width traces and spacing. The image is used to produce two concentric PCB trace spirals, where the acceptable resistance between and end-to-end specifications are given.

In my early days of trying to make PCB, I could only hope for something that actually worked, regardless of the asthetics or (then low) resolution.

Now that I can produce High Resolution Homebrew Printed Circuit Boards almost 100% of the time, I decided to take the Yahoo Group Double Spiral Challenge. I selected the (mid range) "spiral-10mil" Challenge as a starting point. It is 10 mil traces with 10 mil spacing. A quick calculation, suggests that each spiral is about 200 inches (16 feet) long.

2 inch radius / (10 mil trace + 10 mil space) / 2 spirals * 4 inch average turn length ) = 200 inches

Currently, I use a Brother 1200dpi Laser printer (I know, most Toner Transfer users suggest it can not be used). A modified GBC Laminator and Pulsar Transfer Paper.  See previous posts and my Salmoncon Take-Away-Notes.

I recently received a new stock of Double Sided PCB Material and wanted to try this new material.

My first attempt to Transfer an Toner Image, was flawed, a small hair or fuzz was trapped between the transfer paper and the PCB material. That is no real problem, I just re-scrub, reprint and to try again.

My Initial Results

On the second attempt, the Transferred Image looked promising, but the process  had it's share of problems, I must have been just too anxious for the results.

The first thing that I forgot in the process, was to tape over the back side, to avoid unnecessarily etching of the extra copper. The process took much more time and Ferric Chloride than I expected.  I thought this could be a problem.

From my previous Homebrew PCB build process, I know that the faster the etch and the smaller the copper area to be etched, creates the best results. This etch was taking to long!

The results, . . . . FAILURE !

The flaw is on the forth trace
above the ink spot

Resistance between traces implied there were NO shorts. But, end-to-end of each spiral was open. Under close Microscope inspection (and a with considerable amount of time), one spiral was found to be open in one spot, and the other was open in three spots. The one spiral is open on the forth trace, just above the ink dot on the photo.

All of the open flaws appear to be the results of etching through toner pin holes or single point under cutting. The opens did not appear to be the results of general narrowing of the traces. Each opens spot was shorter than the traces width themselves.

I will take the Challenge again, maybe even at a high resolution, but I will pay much more attentions to the details.

For now, I have a nice new, . . coaster.

Update: see http://wa0uwh.blogspot.com/search/label/Spiral

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Printed Inductors

Today I started experimenting with Printed Inductors, which is something that I have always wanted to try. The idea is to print and etch inductors directly on the PCB, many commercial products and some kits use this technique.

For my first experiment I created a 25.5 Turn Inductor with a single 5mil spiral trace, 12mil centre-to-centre spacing, within a 1 inch diameter. The tuning capacitors are soldered to the upper two pads.

 A Tuned Tank Circuit

Note: it is difficult to see the spiral with the naked eye, it is just a blur of shiny copper.

This is the Centre of the Spiral Conductor

My goal was to resonate at my favourite QRSS frequency of 10.140mHz. With the help of a 47pF cap and a parallel 4-50pF trimmer, resonates was obtained on the desired frequency.

I thought the distributed capacitance of the spiral and of the 32mil double sided PCB would kill the Q of the circuit, but it seems to do well as indicated on an AIM-4170B Analyser. But, this will need more investigation.

Scale = 8 to 12mHz

I may have to plan a project using this technique for the tuned inductors.

As I have time, more experiments and information on Printed Inductors will follow!

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