Overview

Currently, any student or researcher can learn how to produce double-sided PCBs with consistent production quality. This is done with the LPKF ProtoMat S43 machine located in Featheringill Hall, room 246. This machine can accurately mill, drill, and route circuit board designs into an FR4 copper-clad substrate with a translational resolution of 0.8 micrometers, but it cannot provide through-hole plating (THP). Figure 2.1 illustrates the production steps for a typical double sided board without solder mask. Figure 2.2 shows the production cycle for a board with solder mask.

Figure 2.1: Default Production Process. drill holes, mill bottom layer, flip, mill top layer, route board outline

Figure 2.2: Solder Mask Production Process.
mill bottom layer, apply bottom mask, flip, mill top layer, apply top mask, flip, drill holes, flip, route board outline

Material

The default material is a FR4 copper-clad substrate covered in 1 oz. copper on both sides with dimensions of 9″ by 12″. Because it is necessary to secure the material by its edges while it is being operated upon, the largest PCB size possible is smaller than 9″ by 12″. Other materials might be used as well, but will require special calibration and cannot exceed 9″ by 12″ in size.

Milling

Milling is how copper is removed from the material to form traces and insulate different parts of your circuit from each other. As shown in Figure 2.3, there are different ways to mill a circuit, each with different levels of insulation. Table 2.1 shows all of the milling tool diameters available, indicating that the smallest allowable space between traces is 0.1 mm.

Figure 2.3: Different Levels of Milling Insulation
The complete rubout method (left) of insulation takes more time and wears the tools harder, but can make soldering easier in the absence of solder mask.

Table 2.1: Milling Sizes

Drilling and Through-Holes

Table 2.2 shows the drill bit sizes available in mm and inches. If the board design calls for a hole that is not matched by one of the drill sizes, and the hole has a diameter greater than 1 mm, then the hole can be drilled as 1 mm and then automatically widened by a routing tool or long end mill tool.

Table 2.2: Drill Sizes

The lack of through-hole plating in the current manufacturing process must be accounted for at the design phase of PCB production. Figure 2.4 shows a cross section of a drilled hole on a double-sided board, and how a through-hole connection might be made by soldering a wire on both sides.

Figure 2.4: Cross section of a hole drilled into a milled copper substrate. The two sides of the board cannot connect unless a wire or component leg is soldered on both sides.

In some scenarios where a through-hole connection is required, the soldered wire connection is not sufficient. This occurs when through-hole components like header pins, dual-inline package (DIP) sockets, or USB connectors are used. Components such as these can only be soldered on one side of the board. There are three ways to fix this problem. The designer may:

• Use a very thin wire to share the hole with the component leg and solder both sides.
• Carefully design the PCB so that the signal-carrying trace reaches the component on the side that it is soldered.
• Use rivets from the LPKF EasyContac kit. Rivets are hollow metal cylinders that fit into an unplated through-hole to connect both sides. Figure 2.5 illustrates the dimension labels of a rivet, and Table 2.3 shows the dimensions of the available rivets.

Figure 2.5: Cross Section of a Formed Rivet
d_i is the internal diameter in which the component must fit.
d_o is the outer diameter of the rivet.
d_d is the through-hole drill size required to fit the rivet.
d_c is the collar diameter which dictates the size of the pad surrounding the through-hole.

Table 2.3: Rivet Dimensions
There are four rivet sizes available.

Contour Routing

The contour router tools are used to cut out a board from the rest of the FR4 substrate when there is no more drilling or milling to be performed. The contour routing tools come in two sizes, 1 mm and 2 mm. Boards may be cut into any shape, even a ring.

Solder Mask

There exists a procedure for applying solder mask to the PCBs created by the ProtoMat. Solder mask is a green lacquer that repels solder and insulates copper traces. Solder mask reduces the need to mill unused copper on the material by simply covering it instead.

For design guidelines and compatability issues, see the Overview and Compatability section of the solder mask guide

Time and Quantity Considerations

The time it takes to produce a single PCB can range from 1 hour to 4 hours, depending on the size and complexity of the board design, the number and variety of holes, the insulation method chosen, and whether or not solder mask is applied. It is possible to create many identical PCBs on a single 9″ by 12″ substrate at the same time, but as production time and material consumption begin to multiply, the benefit of in-house prototyping quickly diminishes. It is recommended to use a PCB fabrication service if many copies of a board are needed, as fabrication services generally become more cost-effective as the order quantity increases.