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The material toolbox idea first came up when I saw the IPC appendix list for standard 1-ply stack-ups. The idea is to make a very simple bill of materials, specifications and notes, and possibly use the same prepreg/resin in the laminate and in the core.
This article will introduce the material toolbox concept by discussing seven important factors:
- Toolbox idea
- Materials concerns
- Technology drivers
- Material needs
In the past, I came across many designers who wished to have the best specifications for impedance. They all start with specifying an impedance and conductor width that will make this according to their field solver. The conductor thickness is given as a constant due to plating or the base copper and the minimum requirement in the hole. This sets the requirement for certain material CCL and prepreg thicknesses. You may see this today in the material supplier’s data sheets for many customers.
If you start with what has the largest input to the impedance in the PCB you will find that it is dielectric thickness and conductor width that contribute the most. Accordingly, I reflected on what could be used as a benchmark for robust design and found that the Big Mac tastes the same anywhere in the world. Not the best burger according to some, but it’s my favorite as it’s something I can relate to and recognize. If I could copy this into our design toolbox I would have a design that, with the right recipe, would re sult in impedance performance of the chosen stack-up that would be the same regardless of where it is produced.
The Big Mac Index provides an estimate of a particular country’s price level. It was first published the magazine The Economist, September 6, 1986. The index is based on McDonald’s Big Mac, which is uniform all over the globe while domestically produced.
You must have a dedicated lane to follow depending on your product requirements. If you choose the mainstream, you have the most options and the lowest price—but the more specific the requirements, the fewer the options and the higher the costs.
Philosophy of Toolbox
The big benefit of standardization is that if you know the impact of the prepreg on the design and you know the impact of copper foil and its different roughnesses you then can understand the impact of PCB manufacturing process tolerances—both the one you can take as default but also how they affect your design and the manufacturing requirements you put onto it. Then you can start comparing differences and design suitable resin systems with your material suppliers.
The toolbox consists of three levels:
- General properties as standard
- Group of materials with the same properties (multiple source)
- Specific supplier data (single source)
When defining this we need to consider who in the supply chain controls the biggest impacts. We see it as follows:
Conductor width - PCB manufacturer
Conductor height - PCB manufacturer
Dielectric height - PCB laminate manufacturer
Dk/Df - PCB laminate manufacturer
By making the choice of specifying standard properties from production variations, the impact of Dk will be less important. Basically, if the tolerances of the dielectric are within IPC B/L (Class 2) and the conductor width for 0.1 mm conductors is +/- 0.03 mm, the impedance will be within 10% without need for measuring. If you choose to make a test board, measure the resulting properties and re-input into your field solver, the control will go to approximately 7%, without further measuring.
For the raw glass material there are a few sources in the world that make it. Many of the extra features with the material make it expensive due to the supply chain locations. Resin is made from oil, and that equals naphtha, a petrochemical product that is a mainstream raw ingredient for the whole industry. Here you must decide between either FR-4.0 (halogenated) or FR-4.1 (halogen-free) to start thinking of the resin’s properties to fulfill its purpose and not over-design the requirements. This helped us a lot going from the 3G products to 4G with its increased requirement for speed. Here it was easy to copy the building blocks of the older stack-ups and reuse them in new materials with better properties.
Weave is important to choose from the cloths mostly produced, and the most produced also got the best tolerances and availability. Higher frequencies require thinner, spread-glass cloths.
This article originally appeared in the May 2018 issue of PCB007 Magazine, click here.