Leland Teschler • Executive Editor Examine advice columns online pitched at engineers and you’ll often see posts advocating an investment in a milling machine specifically designed to produce prototype circuit boards. Engineers who use these machines point to benefits that include a dramatic speed-up in their ability to iterate printed circuit board designs. The usual approach is to order free samples of substrates from PCB suppliers, then mill out a rough prototype PCB which is then populated and tested. Often, having a PCB mill available allows engineers to fabricate and test multiple designs in a day’s time.

The basic function of PCB milling machines is to selectively mill away a copper layer on the circuit board substrate to form traces and other conductive areas on the surface of the board. Some of these specialized machines also have the ability to form vias.

There are, of course, limitations on what can be accomplished on a PCB produced with a milling machine. The primary constraint is that only two-layer boards are possible on simple mills. The creation of a solder mask is also problematic.

In addition, milling machines good enough to produce PCBs can be expensive. The bottom end of the price range for these machines is around $4,000, but more sophisticated equipment able to do vias can be in the $25,000 range. Even more sophisticated laser-based machines get into six figures. These devices use lasers to remove material and thus have some advantages compared to mechanical milling that include higher precision and no drill bits to wear out.

In addition, additive approaches to PCBs are starting to become more practical. These rapid-prototyping machines can be inexpensive alternatives to PCB mills for boards having just a few layers. But there are also a few high-end additive machines said to handle complex high-layer-count circuits as well as electro-mechanical parts.

The Prometheus milling machine from Zippy Robotics.

One PCB mill in the budget-priced category is the Prometheus from Zippy Robotics Inc. in Northport, N.Y. The $3,000 machine has a spindle that runs at 50,000 rpm and has a runout smaller than 2.5 microns. It lists a precision of 0.15625 mils (4 microns) in X and Y, and just 1.25 microns in Z. Its maximum feed rate is 3,800 mm/min, and the machine includes surface tracking said to enable consistent results in cutting depth.

The mill comes with control software called ProCAM that takes industry-standard Gerber files of designs and uses them to plot out up to 6×4-in. board features. Zippy says the machine can create 4-mil trace/5-mil space designs with numerous surface-mount chip packages and do a double-sided board with drilled holes in an hour or so.

A point to note about the 30-lb Prometheus is that is uses special bits only available from Zippy. The company says typical off-the-shelf bits will be slightly larger than the maximum allowable shank diameter and will not enter the spindle. Also of note is that there is no automatic tool-change mechanism.

Bantam Tools in Peekskill, N.Y. once made a desktop milling

The Bantam CNC milling machine. A previous version handled PCB work exclusively. This one also can mill plastic, aluminum, and similar soft metals.

machine optimized for PCB use. That device has been superseded by a more general-purpose implement called the desktop CNC milling machine which can also handle aluminum and plastic parts. The whole machine weighs 70 lb. It has a working envelope of 7 x 9 x 3.5 in. and makes cuts with a repeatability of ±1 mil. The spindle motor is a 0.25-hp unit that spins up to 25,000 rpm. This machine clearly has prototyping applications in mind as the tool-change process is strictly manual.

The Bantam CNC machine lists for about $4,000. Bantam makes a milling machine software package that runs $200 for an annual subscription. Bantam also provides a number of specialized cutting tools for the machine that includes one for PCB engraving.

CNC machines like those from Bantam have general-purpose capabilities that enable them to create PCBs as well as, potentially, the enclosures in which the PCB will sit. But machines from LPKF Laser & Electronics N.A. in Tualatin, Ore. are optimized for making PCBs. The company is perhaps best known for its ProtoMat series machines. There are three models in the ProtoMat lineup which start at about $9,800 and go up to around $27,800.

Top, an LPKF ProtoMat machine. below, an LPKF ProtoLaser machine.

Spindle motor speed in these range from 40,000 to 100,000 rpm, and two of the machines employ automatic tool changing. The faster rpm spindle motors provide cleaner milling edge quality, with less burring. The edge quality is especially noticeable with smaller end mill tools and when milling softer RF substrates. The 40,000 rpm spindle on the ProtoMat E44 allows for a minimum feature size of 4-mil traces with 8-mil spacing. The 60,000 and 100,000-rpm spindles on ProtoMat S64 and S104 systems allow for 4-mil traces with 4-mil spacing. .

Also available on two of the machines are fiducial alignment cameras. These come into play in the fabrication of two-sided PCBs by noting the position of fiducial marks on the board so structures on both sides of the board match up. Higher-end models as well include sensor and software feedback for copper thickness to more precisely control the cut depth, a capability the comes in handy for RF/MW applications.

Another point to note is that some LPKF machines can create multilayer PCBs when teamed up with a lamination press and a through-hole plating kit. There are two levels of sophistication in through-hole kits. The least expensive uses a conductive polymer that’s manually applied to the holes drilled in the board. There is also a special LPKF machine that applies galvanic and chemical processes to plate holes in an enclosed chamber.

Machines in the LPKF ProtoLaser lineup are clearly designed to handle far more than rough prototyping tasks. These use lasers for cutting and shaping operations and range in price from about $91,000 to $373,000. The laser beam ablates the copper next to the future traces without leaving any residue. The resulting structure exhibits sharply cut edges. Typical applications are where precise, steep sidewalls are required or where there’s a need for ultra-fast laser etching, cutting and drilling.

There are four models of ProtoLasers. Among their features are laser beams with a small spot size that makes it possible to cut channels down to 15 μm in width. These kind of precision dimensions are said to be particularly useful for RF applications where striplines are frequently employed.

Where less expensive machines might work well enough for garden variety PCB substrates, laser machines can handle laminated substrates and alumina based ceramics allowing fabrication of fine-pitch boards. The ProtoLaser ST, S4, U4 or ProtoLaser R4 models are often employed for ceramic processing, smaller traces and super-fast metal removal as well as laser etching on select flex materials (U4 and R4). These machines are also billed as providing the most advanced trace/space capabilities with pristine edge definition available without using chemical etch.

High-end LPKF laser machines incorporate a patented hatch and delamination removal process for large copper rubout areas. The laser cuts the area into thin strips and detaches the strips from the organic substrate by heating. This technique is said to drastically reduce the processing time. for sensitive substrates, creating traces below 1 mil (25 µm) wide, pocket engraving and for working with materials such as glass and Teflon or thin-flex substrates.

Other features on the LPKF laser machines include use of a laser wavelength that leaves substrates essentially untouched but which can also reliably process copper surfaces with inhomogeneities up to 6 μm thick. These machines can as well create microvias, and openings in solder masks, cut and structure LTCCs, fired ceramics, ITO/TCO substrates, and delicate prepregs.

Additive 3D printers that can handle PCB work have long been available. Among the principle attractions of this approach is the low cost of the 3D printer. Moreover, 3D printers can potentially fabricate whole multilayer circuit boards complete with solder masks and insulation layers. Some printers also can double as pick-and-place machines to populate the board with components.

Among the primary drawbacks to 3D-printed PCBs is that the conductors are comprised of conductive inks rather than plated-on copper. The resulting traces and pads aren’t as conductive as a conventional copper-plated board—conductive particles only account for about 10%-20% (by weight) of inkjet conductive ink makeup. The lower conductivity may make it impractical to do the same narrow traces available with a PCB mill. Traces made with conductive inks may also have slight variations in height which may cause impedance issues in some high-frequency circuits.

Some 3D printers can’t produce conductive through-holes themselves. The usual practice for making through-holes in these cases is to put the completed board on a drill press and manually drill the hole, then add a rivet that is pushed into place with a rivet press.

A Dragonfly machine from NanoDimension.

Finally, it is worth pointing out that the process of 3D printing PCBs can be faster if the printer can use two print heads simultaneously, one for conductors and the other for dielectric. That’s the approach used by high-end machines such as the Dragonfly line from NanoDimension in Israel. These range in price from $50,000 on into six figures. Besides producing ordinary PCB features, Dragonfly machines can print components that are embedded on the board. Embedded capabilities include printing capacitors, coils, coax, and sensors that detect torque, touch, and strain. DW

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