Hardly anyone lives in a perfect world, except for a small group of people, who call themselves PCB designers.

I have been in the PCB business for more than 30 years. For 25 of these, I have been directly or indirectly involved in design, design rules and specifications. As production technician, I was trying to fulfill the most imaginary specifications. As design support, I was trying to get customers to understand tolerances. As sales or tech-sales, I was trying to get the same message through to the designers over and over. However, I still bump into strange design issues several times a week.

Anyhow, let’s start with the easy, basic stuff; dielectric for inner layers.
When you specify a material, specify only the properties that are important for your board. There is a huge variation of how this is done in practice. The absolute worst-case is brand and grade of the laminate! I understand this in RF or in High-speed digital, but for a standard board this is only limiting your selection of suppliers.

Dk – the second worst
Second worst is, Dk. For the same brand and grade of laminate, Dk varies depending on the thickness or prepregs used in the laminate. So, the Dk is different for 0,075 and 0,5 for the same brand. Then perhaps the most common is to specify Tg. This is understandable.

What board do you want?
You want a board that needs to withstand 3 times lead-free soldering, or a board in a device that has high operating temperature. What often happens, is an “over specification” or some “over engineering” (my favorite word) for it.

As we all know, Tg is giving us a temperature where the physical performance of the epoxy rapidly changes. This change is causing increased Z-expansion in the PCB, among other changes. This Z-expansion is the problematic parameter, if too much, this will cause barrel crack and delamination.

So, in this example we have two different designs. One 0,5mm thick 4 layer and another 3,2mm thick 16 layers. The Z-expansion factor is the same for both, if the same material is used. But due to the thickness difference, the stress in the 3,2mm board is much higher than for the 0,5mm thick.

So, when specifying i.e. Tg 175, most likely both will withstand 3x leadfree soldering and even some repair soldering without a doubt. But did you just put in a cost adder for your 0,5mm board? Would that board have been OK with Tg150 or even with Tg135?

So why specify a Tg? Why not just say, my board needs to pass 3xleadfree process and is function in an operating temperature of xx degrees? Then the supplier can choose a material that is good enough for your conditions. Or why not use the IPC 4101 standard and find a material class that is suitable for your products and specify accordingly?   This approach ensures you get the precise properties you require without limiting your options to a single laminate brand or “painting yourself into a corner.”

Secondly, we have the thickness specification. Check with your PCB suppliers or laminate vendors for available thicknesses in a standard lineup. Use those, and those only. Do not fall for the temptation to add any other thicknesses just to meet a certain thickness in your build.

As for the third case I want to discuss the impedance specified boards. Here we often see very strange specifications. First designer locks the impedances, which is understandable. Next lock is the linewidth to i.e. 100/100. A Third lock is the Dk value. Remember here, Dk values change when thickness of laminate or resin content in the laminate is changing! This can’t be locked.

Everything else is locked, except for the dielectric thicknesses. So, what happens here is that we get very strange laminate and prepreg thicknesses. The outcome of our calculation could be a laminate nominal thickness of 0,313 for instance. I can guarantee there is no such laminate thickness available in the market!   Ending up with a prepreg layer thicknesses that varies within some micros as nominal, is another case, this I will come back to in a later article from Decades of PCB Wisdom.

Check with your PCB suppliers what laminates are available as standard thicknesses.

DO NOT calculate your impedance with the trackwidth as the default, as this might end up with a laminate thickness that is not available.

Please do not just specify 0,313 because it gives the best impedance match with 100/100 tracks. Instead modify your impedance track/gap to match your impedance and choose a laminate thickness that is commonly available from most of the laminate vendors.   Give the manufacturer allowance, in the specification, to modify your tracks within a certain limit without need of rising EQs.

Do not lock your build in 2 ways. Either you can lock the impedances, or you can lock the build, not both! The impedance is often the important one, as this provides some flexibility for the manufacturer to choose the final build and modify track/gap within certain values to match the impedance.

Remember when having impedances, that track/gap is the variable, laminates are fixed values chosen from a laminate list!

I hope you enjoyed your 2 min break, reading and hopefully learning some new points.
In the next post from “Decades of PCB Wisdom”, we will have a look at the prepreg, and dielectric done with prepreg.

Feel free to reach out to me; Terho Koivisto with your questions or design challenges.