Bananas in Rectangles -or- How to save cost in PCB purchase

Read this, if 10% cost savings in PCB bare-board material would be a significant number to your organization.

This article talks about some cost driving aspects in PCB (Printed Circuit Board). Of cause there are many parameters which drive cost of a PCB. I will focus about optimal usage of a PCB inside a panel. In order to get all readers on the same level, let me explain how a printed circuit board gets handled towards manufacturing and where are the cost drivers that I am talking about.

A PCB-Layout Designer will craft the design work. The result is a single PCB as shown.

But a single PCB cannot run thru the manufacturing machines during Assembly/Soldering/Test. Therefore, we put the single PCB several times into a mostly rectangular frame, we call it the “Assembly Panel”. The XY-size is driven by the possible sizes that the manufacturing machines can handle. Which also might vary from product to product being manufactured in the line. Some manufacturing plants try to keep the panel size in one dimension always same, so that they do not need to change the setup of the machines.

Because machinery needs to handle this “rectangle”, we need additional space at the borders top and bottom. Like say 15mm extra. This extra space is also not fixed and can vary depending how I setup my machine (change in setup is cost and time). The single PCB needs to be cut off the panel after manufacturing, we call this “de-panelization”. Obviously, you need room to have eg. milling-tools or sawing-tools to work. So PCBs inside the “Assembly Panel” must have some distance to each other. Following all the required rules, it is some optimization task to optimally place as many PCBs as possible onto a given rectangular shape. Any area, not occupied by your product is waste. The more wasted materials, the more unnecessary cost which gets imposed to you.

What we will order from the PCB bare board fabricator is this “Assembly Panel”. But the bare board fabricator himself, he has other sizes of raw material. Let’s call it the “Fabrication Panel”. The sizes for “Fabrication Panels” do not follow any norm. So their sizes are significantly different among the PCB Fabricators and depening of PCB Technologies like standard, HDI, rigid-Flex.

See PCB fabricators A, B, C, … along with a database containing all possible panel sizes for each fabricator. Notice, the 370mm X 200mm “Assembly Panel” will get at best 88.79% of utilizazion when using FAB Q’s panel of net size 383mm X 434mm and we get two Assembly Panels per Fabricator Panel. So 11% of waste with a cost involved!

We can “play” a little bit with the Assembly Panel size. Reducing the 370mm or reducing the 200mm. Assume we use an Assembly Panel of 355mm x 200mm. Then we recognize a much better utilization of material, 98%. Only 2% of wasted material.

Using FAB A’s available panel of 605mm X 355mm, then we could get three Assembly Panel into one Fabricator Panel, while utilization would be 98%. Obviously, we are interested in high utilization! It saves cost and having less wasted copper, acids, epoxy and other hazard materials going back into the cycle makes feel a bit greener.

Now the question remains, can we design an Assembly Panel within a smaller given size of 355mm x 200mm. The answer is “yes”.

Just turn one of the single PCBs and it will fit on the smaller given size. Now this type of optimization was simple. A human being could easily see, what will be a better positioning. But what if you have complex shapes or a multi-project panel. How to make the best fit. This is where software comes into the game to make a best fit of a “Banana in a Rectangle”.

Optimization tasks:

How do I fit as many “Banana Shaped” PCBs onto a given Assembly Panel?

What size of Assembly Panel will bring me what Utilization at Fabricators A, B, C, …

What combination of all of this gives me least cost?

An optimization task, worth to be done if you are manufacturing your products in high volume. Please bear with me, that I cannot write all the details or recommend particular software tools how to carry out this type of optimization. If you like to talk this topic with me, please do so.

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This is a guest post by Hans Hartmann, Director Sales and Marketing at Zitzmann GmbH, Founder EDA4YOU Sales Rep, Founder DELBOtronik GmbH