Basic knowledge of printed circuit board milling machine

2021-11-10 03:45:43 By : Ms. Ashley Zhao

Leland Teschler • Perform an editorial review for the engineer’s online advice column. You will often see posts advocating investing in a milling machine specifically designed to produce prototype circuit boards. Engineers who use these machines point out that their advantages include the ability to significantly speed up iterative printed circuit board designs. The usual method is to order a free substrate sample from the PCB supplier, then grind out a rough PCB prototype, then fill and test it. Generally, having a PCB factory allows engineers to manufacture and test multiple designs in one day.

The basic function of the PCB milling machine is to selectively mill off the copper layer on the circuit board substrate, forming traces and other conductive areas on the surface of the circuit board. Some of these special machines also have the ability to form through holes.

Of course, there are limits to the work that can be done on PCBs produced by milling machines. The main limitation is that only two-layer plates can be produced on a simple rolling mill. The creation of solder mask is also problematic.

In addition, a milling machine sufficient to produce PCBs can be expensive. The bottom end of the price range of these machines is approximately US$4,000, but more sophisticated equipment capable of vias may be in the US$25,000 range. More complex laser-based machines reach six figures. These devices use lasers to remove material, so they have some advantages over mechanical milling, including higher accuracy and no wear to the drill bit.

In addition, the method of adding PCBs is beginning to become more practical. For circuit boards with only a few layers, these rapid prototyping machines can become cheap alternatives to PCB factories. But there are also some high-end additive machines that are said to be able to handle complex high-level digital circuits and electromechanical components.

Prometheus milling machine from Zippy Robotics.

Prometheus of Zippy Robotics Inc. of Northport, New York, is a PCB factory in the budget price category. This $3,000 machine has a spindle speed of 50,000 rpm and a runout of less than 2.5 microns. It has an accuracy of 0.15625 mils (4 microns) in the X and Y directions, and an accuracy of only 1.25 microns in the Z direction. Its maximum feed rate is 3,800 mm/min, and the machine includes surface tracking, which is said to achieve consistent cutting depth results.

The rolling mill is equipped with a control software called ProCAM, which uses industry-standard Gerber design files and uses them to draw up to 6 x 4 inches. Board characteristics. Zippy said that this machine can create 4 mil traces/5 mil space designs and numerous surface mount chip packages, and complete a double-sided board with drilled holes in about an hour.

One thing to note about the 30-pound Prometheus is that it uses a special drill bit that only Zippy can provide. The company stated that a typical off-the-shelf drill bit will be slightly larger than the maximum allowable shank diameter and will not enter the spindle. It is also worth noting that there is no automatic tool change mechanism.

Bantam Tools of Peekskill, New York used to do desktop milling

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

Machine optimized for PCB use. This equipment has been replaced by a more versatile tool called a desktop CNC milling machine, which can also handle aluminum and plastic parts. The whole machine weighs 70 pounds. It has a working area of ​​7 x 9 x 3.5 inches, and a cut repeat accuracy of ±1 mil. The spindle motor is a 0.25 horsepower device with a speed of up to 25,000 rpm. This machine obviously allows for prototype applications, because the tool change process is strictly manual.

Bantam CNC machines sell for approximately US$4,000. Bantam has produced a milling machine software package with an annual subscription fee of $200. Bantam also provides many special cutting tools for the machine, including a tool for PCB engraving.

CNC machines like Bantam have common functions that enable them to create the PCB and possibly the enclosure in which the PCB is located. But the machines at LPKF Laser & Electronics NA in Tualatin, Oregon are optimized for PCB manufacturing. The company may be famous for its ProtoMat series of machines. There are three models in the ProtoMat product line, with prices starting at approximately US$9,800 and up to approximately US$27,800.

At the top, the LPKF ProtoMat machine. Below is an LPKF ProtoLaser machine.

The spindle motor speed ranges from 40,000 to 100,000 rpm, and two of the machines use automatic tool change. The faster spindle motor provides cleaner milling edge quality with fewer burrs. The edge quality is especially noticeable when using smaller end mills and milling softer RF substrates. The 40,000 rpm spindle on the ProtoMat E44 allows a minimum feature size of 4 mil traces with 8 mil spacing. The 60,000 and 100,000 rpm spindles on the ProtoMat S64 and S104 systems allow 4 mil routing at a 4 mil pitch. .

Two of the machines also have fiducial alignment cameras. By paying attention to the position of the fiducial marks on the board, these play a role in the manufacture of double-sided PCBs, so the structure on both sides of the board matches. Higher-end models also include sensors for copper thickness and software feedback to more accurately control the cutting depth, which is very useful for RF/MW applications.

Another point to note is that some LPKF machines can create multilayer PCBs when used with laminators and through-hole plating kits. The through-hole kit has two levels of complexity. The cheapest method is to use a conductive polymer that is hand-applied and drilled into the board. There is also a special LPKF machine, which uses electroplating and chemical processes to electroplate holes in a closed room.

The machines in the LPKF ProtoLaser series are clearly designed to handle much more than rough prototyping tasks. These use lasers for cutting and forming operations, and prices range from US$91,000 to US$373,000. The laser beam will ablate the copper next to the future trace without leaving any residue. The resulting structure exhibits sharp cutting edges. Typical applications are where precise and steep sidewalls are required or where ultra-fast laser etching, cutting and drilling are required.

ProtoLasers has four models. One of their features is a laser beam with a small spot size that can cut channels as small as 15 μm in width. This precision size is said to be particularly useful for RF applications where stripline is often used.

Cheaper machines can handle all kinds of PCB substrates well, while laser machines can process laminated substrates and alumina-based ceramics, which can make fine-pitch boards. ProtoLaser ST, S4, U4 or ProtoLaser R4 models are commonly used for ceramic processing, smaller traces and ultra-fast metal removal, and laser etching on selected flexible materials (U4 and R4). These machines are also advertised as providing state-of-the-art tracking/spatial capabilities and raw edge definition without the use of chemical etching.

The high-end LPKF laser machine uses a patented hatch and delamination removal process, which is suitable for large copper abrasion areas. The laser cuts the area into thin strips and heats them to separate the strips from the organic substrate. It is said that this technique can greatly reduce processing time. Suitable for sensitive substrates, creating traces with a width of less than 1 mil (25 µm), pocket engraving, and processing materials such as glass and Teflon or thin flexible substrates​​​

Other features of the LPKF laser machine include the use of laser wavelengths that leave the substrate basically unaffected, but can also reliably process uneven copper surfaces up to 6 microns thick. These machines can also create micro-holes and openings in the solder mask, cut and structure LTCC, fired ceramics, ITO/TCO substrates and fine prepregs.

Additive 3D printers that can handle PCB work have long been available. One of the main attractions of this approach is the low cost of the 3D printer. In addition, 3D printers have the potential to manufacture complete multi-layer circuit boards with solder mask and insulation layers. Some printers can also double as pick-and-place machines, filling circuit boards with components.

One of the main disadvantages of 3D printed PCBs is that the conductors are made of conductive ink instead of copper plating. The resulting traces and pads are not as conductive as traditional copper-plated plates-conductive particles account for only 10%-20% (by weight) of the inkjet conductive ink composition. The lower conductivity may make it impossible to use the PCB factory for the same narrow traces. Traces made of conductive ink may also have slight changes in height, which may cause impedance problems in some high-frequency circuits.

Some 3D printers cannot produce conductive vias by themselves. The usual way to make through holes in these situations is to place the finished board on a drill press and drill manually, and then add a rivet that is pushed into place with a riveting machine.

NanoDimension's dragonfly machine.

Finally, it is worth pointing out that if the printer can use two print heads at the same time, one for the conductor and the other for the dielectric, the process of 3D printing the PCB will be faster. This is the method used by high-end machines, such as the Dragonfly series from NanoDimension in Israel. These prices range from $50,000 to six figures. In addition to producing ordinary PCB functions, Dragonfly machines can also print components embedded on circuit boards. Embedded functions include printing capacitors, coils, coaxial cables, and sensors that detect torque, touch, and strain. database

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