Mechanical Circuit-Board Plotter Turns Out Complex PC Boards Quickly

Mechanical circuit-board plotters speed up the electrical design process by allowing designers to build pc-board prototypes in a matter of hours. For designers facing time-to-market pressures, that speed can be critical in completing a project on time. Without in-house board prototyping capability, designers must rely on quick-turn service bureaus that may not meet their schedules. Board plotters also offer a cleaner, more advanced alternative to lab etching tanks, which may lack the accuracy to fabricate more advanced layout designs.

Under computer control, mechanical circuit-board plotters use carbide tools to mill the pc-board artwork, to drill holes on FR4 substrates and other laminates, and to route the contours of the board. Designers employing these plotters in combination with in-house plating and lamination equipment can readily build reasonable-size pc boards with up to four layers in one day. However, one-day turnarounds for six-layer boards are out of reach for all but the very smallest board designs. That reflects the equipment's current limits on milling speed as well as the fixed time intervals required for plating and lamination.

Now, a new circuit-board plotter from LPKF ups the ante for same-day turnarounds, enabling designers to fabricate reasonable-size prototype boards with six to eight layers. Because the time required to build a board is more a function of its number of features than its overall dimensions, consider "reasonable size" to mean pc boards containing about 1000 via holes. The ProtoMat H100 achieves its improvement in productivity by increasing milling speed and eliminating the need for user interaction associated with tool alignment and positioning of artwork. Simultaneously, the new system advances the board plotter's accuracy, improving milling resolution by an order of magnitude over existing equipment (see opening figure and Table 1).

Faster Operation: To accomplish this, LPKF had to speed up the milling process because other parts of the board prototyping process—the plating and lamination stages necessary to build multiple layers—require fixed amounts of time. The Protomat H100 uses a combination of advances to increase the board plotter's average milling speed across an entire pc board by two to three times.

A primary reason for this progress was an increase in the milling motor's performance. For the past 10 years, this type of motor's operating speed has been limited to 60,000 rpm to maintain an acceptable motor cost. Yet ProtoMat H100 takes advantage of a newer motor type that's capable of 100,000-rpm speeds. This style of motor contains ceramic bearing materials that make the high-speed motor less expensive to produce, while also ensuring sufficient operating life. The faster milling motor lets the plotter take advantage of the faster performance achieved by the plotter's board positioning system (the X-Y table), which the system's X-Y drive control determines.

Within the drive control system, several elements influence performance. Movement of the X-Y table is driven by precision ball screw assemblies with quiet, nearly resonance-free three-phase stepper motors. These motors deliver approximately 50% more torque than standard stepper motors of comparable size. As a result, the ProtoMat H100's steppers provide faster acceleration and movement of relatively high masses.

The ProtoMat H100's X-Y motion controller operates these motors in a microstep mode with a maximum frequency of 10 kHz, resulting in a resolution of 1 µm per microstep. The motion controller optimizes acceleration and deceleration based on each vector—the movement from one set of pc-board coordinates to another. Meanwhile, the ball screw assemblies contribute to the board plotter's high positioning accuracy, while preventing backlash.

Other improvements in the drive control include a precision low-friction rail system. The drive system achieves a maximum positioning speed of 4 in./s versus 2.4 in./s for existing circuit-board plotters.

These features are combined with an intelligent sorting of milling coordinates and what LPKF calls "following-vectors" processing to maintain optimum milling speed even on short vectors. The following-vectors algorithm keeps the motion of the X-Y table at an optimum speed as the motion controller anticipates the coming moves. In other words, the controller optimizes the acceleration and deceleration of the stepper motors based on the angles of the next vector. The smaller the relative angle of the next vector, the higher the speed.

Because the stepper motors must move significant masses, the maximum acceleration of the table itself is limited. Consequently, an increase in acceleration has a great impact on the average milling speed of the plotter, especially because most pc boards contain many small features that don't allow much distance to accelerate.

Compare this technique to how previous mechanical pc-board plotters stopped after each vector to prevent overshooting. That forced the plotters to accelerate from zero after every turn. Just by optimizing acceleration and deceleration ramps, the following-vectors technology increases the average milling speed of the plotter by 20% to 30%.

For the ProtoMat H100 to achieve its higher plotting speed, it was necessary to speed up the interface used to load data into the plotter. The 9600-baud RS-232 serial interface, which is found on existing board prototyping equipment, was simply too slow. The ProtoMat H100 includes a USB 1.1 connection running at 12 MHz. However, the plotter also features a standard RS-232 connection and Windows compatibility.

Automatic Alignment: The ProtoMat H100 further improves plotter performance by automatically calibrating milling depth. Until now, mechanical board plotters could change mill bits automatically, as in this plotter. But the equipment operator had to manually align the milling bit's depth after each change. In the H100, an optical sensor mounted in the machine's tabletop enables a software-controlled motorized adjustment of the depth to an accuracy of 5 µm (0.2 mils).

Alignment procedures are automated still further using a camera vision system mounted to the milling head. The camera provides automatic recognition of fiducials, such as crosshairs or drill holes, used for alignment. Typically, the fiducials are buried within the pc-board stack under layers of copper and FR4 material. The ProtoMat H100 locates these markers by identifying the general areas where they're located, then uses an endmill (a flathead milling tool that has a rectangular cutting profile) to remove the copper foil and FR4 in those areas. Next, the camera locates the fiducials and locks onto them. Though this process is commonly used in high-volume pc-board manufacturing, it was never before applied in prototyping equipment.

The ability to locate fiducials in this way ensures proper alignment of the pc board after it has been removed from the plotter for lamination or plating, or after the pc board has been turned over. It prevents gross errors, such as milling the wrong side of the pc-board stack when the board has been placed back on the machine upside down or if the board has been rotated 180° out of proper alignment. These errors could occur after the board stack undergoes lamination. By preventing the plotter from milling the outer layers of a misaligned pc board stack, this alignment capability prevents the engineer from ruining the pc boards at the point where the prototypes are nearly done.

Proper location of the fiducials also compensates for slight errors in board misalignment that would compromise the plotting accuracy of the equipment. In part, this lets the equipment achieve its higher levels of resolution than existing board plotters. Moreover, the combination of automated board alignment with automated depth adjustment of the mill bit eliminates user interaction to the point where the plotter may be left to run unattended overnight.

Higher Precision: The ProtoMat H100 achieves a plotting resolution of 1 µm (0.039 mils), which is five times more precise than existing state-of-the-art circuit-board plotters and 10 times more accurate than some standard equipment. That resolution permits high circuit densities with tracks of 75 µm (3 mils) or less with 100-µm spaces. Until now, mechanical board plotters were limited to minimum track widths of 100 µm.

The increased accuracy enables the plotter to route more interconnects within a given area, while accommodating almost all state-of-the-art fine-pitch packages, including advanced BGAs (Fig. 1). The resulting increase in circuit density could potentially save layers in a multilayer board design.

The accuracy of the system reflects the high resolution of the X-Y positioning system, the consistency and accuracy of the milling depth control, the use of custom-made milling tools, and the milling motor's high speed. LPKF uses a 60° V-shaped spade tool to mill very small lines, while still maintaining a reasonable tool life of over 700 in. To extend the life of this specialty tool, the board plotter's CAM software contains a smart-tool function that uses this tool only where it's necessary. As soon as spacing allows, the software automatically switches to a standard tool. Normally, the standard tool has eight to 10 times the operating life of the spade tool (Table 2).

Plotter Operation: Current tests indicate that the ProtoMat H100 takes an average 50% less time to mill a pc board than existing equipment. On a common six-layer board, milling time would be reduced from about eight to 10 hours to just four hours. When additional operations for drilling (30 minutes), plating (60 minutes), and lamination (90 minutes) are factored in, the same six-layer board and (perhaps even eight-layer boards, depending on size) can be built in one day.

To build a six-layer board, the plotter carries out a six-step process (Fig. 2). In steps 1 and 2, the plotter mills all the inner layers (2 through 5) on double-sided board material. In milling the three boards a, b, and c (Fig. 2, again), the plotter doesn't mill the bottom side of board c or the top side of board a in steps 1 and 2. These surfaces, which become the top and bottom sides of the stack, will be milled in a later step.

In step 3, the three double-sided boards are laminated to create the final board stack using LPKF's MultiPress II lamination press. Press parameters including time, pressure, and temperature depend on the prepreg material (unclad FR4 material in an uncured state) employed and are specified by the prepreg material supplier. Following lamination, the plotter drills through-holes (step 4) using its vision system to align the artwork with the board. In the same step, LPKF's plating tank equipment can be used to connect inner and outer board layers. Finally, in steps 5 and 6, the outer layer artwork is milled again while relying on the vision system once more to align the artwork with the multilayer board after it is repositioned on the plotter.

LPKF Laser & Electronics, U.S.A., 28220 S.W. Boberg Rd., Wilsonville, OR 97070; Stephan Schmidt, (503) 454-4200; [email protected]; www.lpkfusa.com.