Clifton Laboratories 7236 Clifton Road  Clifton VA 20124 tel: (703) 830 0368 fax: (703) 830 0711

E-mail: Jack.Smith@cliftonlaboratories.com

Click to jump to the section:
 
Introduction_
Solderless_Board_for_Digital_and_PICs
Manhattan_Construction_for_RF
BNC_Mounting_Brackets
Island_Pad_Prototyping
Pad_Capacitance_
Teflon_Pin_Inserts
Z90_Prototype_
Analog_Panadapter_Construction_with_Inverted_DIP_Prototype_Boards

As convenient as solderless plugboards are, they leave a lot to be desired for RF prototyping and, for that matter, high speed digital logic as well. Fortunately, there's an excellent and simple RF prototyping solution—Manhattan style construction.

I don't know who invented Manhattan-style construction, but for a good summary of the technique it's hard to beat K7QO's article at http://www.k7qo.net/manhattan.pdf. K8IQY has also made good use of Manhattan-style construction in its projects as illustrated at his web site http://www.k8iqy.com/index.html.

As convenient as these small PCB holders go, there's a limit to how much you can jam into a couple of square inches, particularly when you wish to spread parts out for easy measurement and rework.

There's no limit on how large a piece of PCB stock you use, of course, but it's inconvenient to have RF connections made via soldered coax or other jury-rigged techniques. I've made a few BNC mounting brackets to solve that problem.

Manhattan-style construction has one persistent problem—heat from soldering breaks the superglue bond. I've found 5-minute epoxy is a partial solution, but soldering heat eventually separates even epoxy if you work on a pad enough.

One alternative developed by the New Jersey QRP Club was the "Islander Pad Cutter," no longer offered for sale, but the documentation is still available at http://www.njqrp.org/islanderpadcutter/index.html. The Islander Pad Cutter is a diamond "core drill" that, when used with a drill press and a light touch will cut an annular ring, leaving an isolated round pad attached to the PCB. Diamond core drills are widely used by stained glass and lapidary hobbyists and, for the size one would want for pads, are not very expensive. For example, http://www.diamond-drill-bit-and-tool.com/Diamond-Drill/MAIN.htm carries suitable drills  in the $14-$16 range.  I've seen new diamond core sets on E-bay for a fraction of that price, so shop around if you decide to purchase one.

I made an island pad cutter from a short length of 0.25" diameter "W1 water hardening drill rod" on the lathe in a few minutes. Face one end of the drill rod, and center drill it. Then, with a No. 4 bit, drill approximately 0.5" into the rod. With a center drill, carefully drill the hole to a thin edge.  With a suitable stone, smooth the outside and inside. Heat to orange and quench in water. Stone again if necessary.

It is not necessary for bit's cutting edge to have teeth or to be rough. In fact, when I tried filing notches in the cutting edge, I found that 95% of the pads I tried to cut ripped the copper right off the PCB surface.

Chuck the rod in your milling machine or drill press and set the depth stop so that the bit will just cut below the copper layer of your PCB. The speed should be set high, 2000 RPM or more. Although I have not tried it, I suspect it will be somewhere between difficult and impossible to consistently cut pads using a hand held drill. Use a drill press and adjust the depth stop so that you don't have to guess.

Note—when you use a tool such as the island cutter with a piece of thin stock, THINK SAFETY. You don't want to hold the PCB stock with your fingers and have the cutter dig in, spinning the stock and sending you to the emergency room to have your fingers stitched back together! Use a drill press vise or C clamps or other tooling to hold the PCB in place. DO NOT USE YOUR FINGERS!

5 mm or 3/16"
7 mm or 1/4"
8 mm or 5/16"

I found these drills yield approximate pad sizes as follows:

The pad island diameter will vary to some degree depending on how deep you press the drill. You can see this effect in the photo below, with the two pads cut with a 1/4" drill. The pad at the far right has a slightly larger island than the center pad.
 

And, as I've said before, please use good safety practices—clamp the PCB to the milling machine or drill press  table and don't hold it with your fingers. The diamond bit will grab the PCB and spin it around without notice, placing your fingers at risk. Don't do it -- use a clamp!

Geoff, GM4ESD, has asked how much capacitance exists between an island pad and ground, assuming the pad is cut on a piece of double-sided PCB stock and that the top and bottom copper planes are tied together, as is normally the case. This question also applies equally to a standard Manhattan-style construction with PCB pads glued to PCB stock.

I'm temporarily out of 0.062" double-sided PCB stock, so I ran tests on thinner material I've recently used. This PCB stock is 0.0299 inches (0.759 mm) thick, measured with a digital micrometer accurate to 0.0001.". My original assumption was that the stock has 1 oz. copper, a very common value, is 0.0014" (0.036 mm) thick. Graham, KE9H, wrote to correct this assumption, saying that it's almost certainly 0.5 oz foil  in its present form, as the additional copper for a 1 oz delivered finish is supplied during the electroplating process (traces and holes) as the raw stock is made into a completed PCB. Hence, instead of 1.4 mils copper thickness, it's 0.7 mils. (0.036 mm and 0.018 mm, respectively.) I've accordingly corrected the calculations on this page. (I also re-measured the PCB stock  thickness at five locations and use the average, 0.0299" in the calculations.)

If so, the fiberglass dielectric thickness is thus 0.0285 inches (0.724 mm) .

The material appears to be FR-4 fiberglass, the dielectric constant of which ranges from 4.2 to 5.2, with 4.7 used as a nominal center value. However, the dielectric constant is a function of humidity, temperature and frequency and a study by Dupont shows measured dielectric constant for FR-4 material as high as 5.82 under conditions of high humidity and elevated temperature, and no values below 4.52.
 

To determine the board's dielectric constant, I measured the capacitance between the top and bottom foils using a DCM-601 digital capacitance meter.  The board sample is rectangular, with a total area of 2.681 square inches, or 1730 mm² if I've done my metric conversion correctly, and measured 110.4 pF.

The formula for a parallel plate capacitor is:

where
C is the capacitance in Farads
ε is the effective permittivity consisting of ε₀ x ε_R
ε₀ is the permittivity of free space, 8.85x10^-12 F/meter
 ε_R is the relative permittivity (dielectric constant) of the PCB material
A is the plate area in square meters
d is the separation between plates in meters

I've recast this formula into Imperial engineering units as:

Cpf = 222 * A * ε_R / d
where
A is the area in square inches
d is the separation or PCB fiberglass thickness in mils (0.001" units)
 ε_R is the dielectric constant.

If  ε_R is 4.7, the "average" value for FR-4 PCB stock, then this equation can be further simplified as:

Cpf = 1041 A/d

For back-of-the-envelope engineering purposes, the capacitance in pF for FR-4 material can be simplified to an easier-to-remember number of 1000 pF * area in square inches / thickness in mils.

Using the exact parallel plate capacitance formula, working backwards, from the measured capacitance and board area, I computed  ε_R as 5.22 for my PCB stock.  This is at the edge of the nominal 4.2 to 5.2 range, but within the range measured by Dupont under conditions of high humidity. Remember that  ε_R varies between different board samples, and also changes with temperature, humidity and frequency, as well as the ratio between glass fibers and epoxy in the material.

This calculation assumes the "fringing" capacitance is negligible. Fringing capacitance is from the electrostatic lines of force at the perimeter of the board that are in air. I don't have a good estimate of the fringing capacitance, but expect it to be at most a couple of pF for this size board. If fringing capacitance is considered, then the true parallel plate capacitance is less than measured and hence the computed dielectric constant will correspondingly decrease. (I am aware of free finite-element electrostatic simulators available on the Internet, and will leave it to the interested reader to duplicate the  test setup with such simulation software. The simulation software will consider and compute the fringing capacitance.)
 

We can easily calculate the capacitance of an island pad, using the equations above. In this case, with ε_R =  5.22, and with a board thickness of 0.0285" we calculate C = 41.18 pF / in².  (This number could, of course, be more simply derived by dividing the measured sample capacitance, 110.4 pF, by the sample area, 2.681 square inches.)

A single pad, cut with the 8 mm (5/16") diamond core drill leaves a pad diameter of about 0.29." The exact pad diameter varies with how deep the drill is pushed and if the PCB is not perfectly flat, the annular insulating area will not be of uniform width.

For a 0.29" diameter circular pad, we compute the capacitance as 2.72 pF. To this value should be added an allowance for fringing capacitance, i.e., the capacitance from the pad to the top surface foil across the annular insulating area.  Again, I don't have a good estimate for this fringing capacitance, but there is reason to believe it's rather small, as will be seen shortly.

I've measured the capacitance of a 0.29" diameter pad two ways. First, using eight pads in parallel, and second, a single pad.
 

To reduce the effect of the cage, I tried a different approach, as illustrated in the photo below. I cut three pads into the PCB as seen in the photo. I mounted the board on a Boonton RX Meter, model 250A, set for 1 MHz measurement. I moved the wire until is was just a hair short of contacting the pad and nulled the 250A RX meter. The null adjustment therefore considered and cancelled the capacitance of the wire contact to the PCB and the instrument. I then moved the wire contact a tiny amount to contact the pad and adjusted the 250A's X dial to restore bridge balance. I measured 2.80 pF. The rated accuracy at this frequency and dial reading is ±0.165 pF. The RX 250A meter is calibrated in 0.01 pF increments and easily meets this accuracy.
 

Calculated: 2.72 pF
Measured: 2.80 pF ±0.16 pF.

This is excellent agreement and suggests that the fringing capacitance for this size pad and board thickness is small, at most being 0.24 pF, and a mean value of 0.08 pF. And, my pad diameter measurement is only an estimate, made with a digital caliper held against the pad. A small error in pad diameter will have a 2x larger effect in capacitance error, since capacitance is proportional to the area (diameter or radius squared).

The remaining question is "is 2.8 pF capacitance too much?"  There's obviously no single answer to that question, as it depends on the point in the circuit and the frequency. Moreover, if 2.8 pF shunt capacitance is too large, there are several strategies to mitigate it:

1. Use a smaller pad. If cut with a 5 mm (3/16") core drill, the pad diameter is 0.15" and hence the shunt capacitance to ground will be on the order of 0.7 pF.

2. Use 0.062" thick PCB stock. In fact, I prefer the 0.062" board, but happen to have used all I had on hand at the moment. 0.062" PCB stock will have only about 40% of capacitance I measured with the 0.0299" material. A 0.15" diameter pad on 0.062" stock will thus have about 0.28 pF shunt capacitance. In this case, fringing capacitance may become an important factor and should be considered.

3. If your circuit otherwise permits, use single sided PCB stock. This will reduce the capacitance to nearly zero, but may or may not cause other problems.

4. Mount the sensitive portions of the circuit in air, spaced above the PCB. In some cases, high value resistors may be used as improvised stand-offs. Or, the purpose-made Teflon insulated terminal pins, such as those from Keystone Electronics, 11065 - 11074 series, with pin heights up to 1." The Teflon base press-fits into a hole in the PCB and the pin provides an insulated structure, above the PCB base. Or, one might glue an inverted DIP socket to the PCB and use the pin leads. (I've seen this done with octal sockets years ago.)

I should also add that using conventional Manhattan-style construction has exactly the same issue—there is shunt capacitance from the pad mesa to the PCB surface, which will be similar to the shunt capacitance of an island pad arrangement with the same dimensions and thickness. 
 

One problem with either island pads or glue-on Manhattan pads is capacitance to the ground plane. For many circuits, the small additional capacitance is not a problem. But, it can be for some circuits. If you are building an oscillator, even if 1 or 2 pF of pad capacitance isn't a problem by itself, changes in the pad capacitance can result in frequency instability. Standard FR-4/G10 PCB material is not known for high dielectric constant stability, after all, as it changes with both temperature and humidity.

One alternative mentioned in the the text above this topic, is to build sensitive portions of the circuit on Teflon stand-off pins. I purchased a couple hundred pins from a surplus dealer on E-bay recently and recently used them for the critical parts of a crystal oscillator circuit.
 

Close up of pins around the oscillator stage.

My Z90 started out with a combination of plugboard and Manhattan-style construction. The microcontroller and interface to the graphics LCD were constructed on a solderless plugboard, and the rest of the circuit on a series of Manhattan-style modules. Each module was first developed and tested with a test fixture like the one shown above, or one of a similar design I later built.

I've subsequently made changes in that original design, and PCB layout, to make it easier to build (for example, moving the DDS to a small daughter board) and to improve performance. But, I could make those changes with confidence that each module was a functioning unit with stable interfaces.