OBSERVATIONS FROM SCOTLAND 28 May 2008 GM1SXX
Copper Island Construction.
Some of us like to dabble, to build various circuits and prototypes. In general, it's not cost effective to make a PCB for a 'one-off' so alternative construction methods have to be sought. Depending on the type and complexity of the circuit, the common methods include VEROboard, 'ded-bug', various types of prototyping boards, solderless breadboard or my favourite.. copper island, AKA 'Manhattan construction' method.
'Copper Island' construction is a simple idea for producing neat tidy prototypes. You simply use 5 or 6mm diameter disks of material punched out of single-sided SRBP board as anchor points for the components. The disks are cyano-glued to a sheet of single sided PCB in the appropriate places and tinned to provide insulated fixings for the circuit components. It's a particularly good system for beginners because it's easy to lay out circuits to mimic the circuit diagram although experienced constructors will tend to save space by cramming more parts onto a given area of PCB.
The copper island construction method normally used round disks punched from single-sided PCB material although some DIY'ers use little squares instead. It's not easy to hand-cut very consistent squares, so I instead opted for a hand-punch system that would give me consistenly sized disks of material. 5mm diameter seems to be an ideal size although 4 and 6mm are also perfectly suitable for most work.
There seems to be no commercial supplier of 'copper island' disks so I set about finding a way to make them myself.
To this end, I purchased a hand punch from Ebay. It came with a number of punches and dies but the punch parts have a little central 'pip' to avoid the work-piece sliding around as the holes are being punched. This effectively made the punch useless for my needs. I wanted to punch out perfect flat circles of PCB and the punch was destroying the material I wanted to keep!
My solution was to modify the punch set in the photo by grinding off the pointed ends of the punch parts in order to produce perfectly round disks of PCB material as shown. I used a Dremel machine with a cut-off disk to re-shape the end of the punch to a flat surface. This allows me to produce perfectly flat 5mm diameter disks of PCB material.
I used single-sided SRBP circuit board rather than the more common Fibreglass material. This is easier to work and kinder to the tool. Fibreglass PCB is rather 'cruel' to cutting tools!
The Copper Island method of prototyping can be used from DC to at least 23Cms if due care is taken over layout. This make it very attractive to the home constructor looking to build moderately complex prototypes. It's even possible to use the larger SMD components with copper island. In fact, I use mainly SMD in my prototypes. Should you need to fit an SMD decoupling cap, it can simply be leaned against a pad at a forty-five degree angle and soldered to both pad and groundplane. By placing adjacent pads at a suitable distance apart, resistors and capacitors can simply straddle adjacent pads. I regularly use a mix of leaded and SMD parts in the same circuit. To be perfectly honest, I much prefer to use SMD rather than leaded components.
Small SMD components such as MMIC's can be fitted to a single copper pad after a Dremel machine and 'cut off disk' are used to grind a cross shape into a standard pad so the device can be placed on it and soldered. Glue the pad to the board first and hold the board firmly in a vice while grinding the 'cross' into it... and wear eye protection. Safety first!
Should you have access to a hand punch with different sized heads, a larger than standard pad can be made for short leaded parts such as ga-asfets and MMICs. If you do use this mounting method, make sure your ground leads are as short as practical. Because of their high gain, MMIC's and in particular, PHEMTS are *very* fussy about the length of gound connections!
An even better method of mounting MMICs is to bend both ground leads carefully downwards, solder to a pre-tinned spot on your pcb, gently bend input and output leads upwards and slide glue-laden copper island pads under the input and output leads, then solder in place. This method results in the lowest stray capacitance.
Be inventive! Who knows, perhaps YOU could come up with something even better than copper island.
73 AL.
GM1SXX