Maximizing Panel Utilization

Let’s say you go to your favorite grocery store looking for some ready-made meals for a quick breakfast, lunch, or dinner. You’re near the deli counter and find you can buy one sandwich for $8, or a pack of four of the same sandwich for $20, or $5/sandwich when you buy in volume. Being frugal, and needing to eat four or more meals this week, you buy the pack of four. The next day you eat one of the sandwiches for lunch, and one the day after that. On the third day, you decide you don’t want a sandwich, and coincidently work has a catered lunch. You forget about the two leftover sandwiches only to find them a week later growing mold in your refrigerator. 

In this example, you bought the sandwiches for $5/sandwich, but because you had to throw two of them out, you paid $10/meal. You would have been better off buying one for $8! Panel utilization is similar in that when ordering a panel of PCBs, you pay for the material of the whole panel, including the parts that get thrown away. Might as well use as much of it as possible! It’s important to consider this early in the design stage as once the mechanical elements are defined, it may be cost-prohibitive to change the PCB dimensions in a cost reduction due to the amount it costs to change the mechanical structure and associated tooling!

The first step in this process is to call your production PCB fabrication shop to find out what their standard panel sizes are. It’s a good idea to call your prototype PCB fabrication shop also to find the overlapping panel sizes, but if you’re ordering in small volumes from the proto shop, it’s not as critical. Generally, this is only needed once per vendor, as the stock they carry shouldn’t change that often, especially if you stick with their most common sizes. 

Next, you need to understand what the requirements are for the border and PCB separation. A typical PCB panel will have some border needed for the pick-and-place machine to carry it through the process. The border material is necessary for assembly but doesn’t serve as a functional part of the PCB once the boards are assembled, so it will (most likely) end up as waste. Likewise, if your PCB is smaller than the usable space in the bare fab, there will be some additional space required to separate the individual PCBs in the fab as the cutting blade has a non-zero width. This is typically done with routing (removing part of the panel) and V-scoring (creating a section in the panel that’s easy to break away). 

Let’s visualize: 

In the image above, the stock width and height are represented by W and H, respectively. The usable width and height of the stock are represented by W’ and H’, respectively. The space required between each individual PCB is referred to as ‘Board Separation,’ labeled ‘c.’ The border thickness we’ll call ‘B’ (not labeled). Each individual PCB has a width of WB and a height of HB. The panel is laid out so that there are ‘m’ PCBs across the width, and ‘n’ PCBs along the height. 

The maximum board dimensions for a given number of boards along the width (m) and height (n) is calculated using the formulas below. Where m, n are integers. 

Let’s break this down into some formulas: 

The percent utilization of the stock can be calculated based on the number of PCBs along the width (m) and number along the height (n), the usable width (W’) and height (H’) of the stock, and the with (W) and height (H) of the stock using the formula: 

With the formulas above, one can create a table showing the dimensions of a PCB that maximize the percent-utilization for a specific number of boards across the width and height of the stock. One common stock size is 24”x18”, with a border requirement of 0.75”, and a board separation of 0.15. With those numbers, we can calculate PCB sizes that maximize the percent utilization for different numbers of boards per panel. 

Given W=24”, H=18”, B=0.75”, C=0.15”, our goal is to find the PCB size (WB x HB) that maximize the percent utilization of the stock for various combinations of m and n. The table below shows the dimensions of a PCB that maximize the percent utilization of the stock, and what the percent utilization is. 

The table above shows the number of PCBs across the width (m, cyan) and the associated width dimension of the PCB (WB) along the top. Along the left side, it shows the number of PCBs across the height (n, purple) and the associated height dimension of the PCB (HB). The data in the table shows the associated percent utilization of the fab for a PCB with dimensions WB x HB. 

For example: For the stock mentioned above, with the required border and board separation, a panel with 5 PCBs across and 5 PCBs up, the dimensions that maximize the percent utilization of the stock are 4.380” x 3.180”, with a utilization of 80.6%.

When starting the design of a product, it’s good to take PCB size into consideration as it’s not something that can necessarily change easily when attempting to cost reduce the product. Once the mechanical shape is defined, and molds made, modifying those can outweigh the cost savings from changing PCB dimensions. Taking the percent utilization of a fab into consideration early in the design cycle can influence the mechanical design and solve the problem of changing the mechanical structure to allow for a cost reduction of the PCBs. 

There are many other considerations to take into account when sizing a PCB. For instance, the mechanical design may drive a certain dimension range or there may be a physical limitation to the product that forces a PCB shape that is not rectangular. Of course, there are other aspects that one may not see coming when choosing PCB size this way, such as in the example above, the 4.380” dimension might create a great quarter-wavelength antenna for about a 700MHz signal, and how a 3.180” dimensions might create a great quarter-wavelength antenna for about a 900MHz signal, but that’s a story for another time.