PCB007 Magazine

PCB007-June2019

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JUNE 2019 I PCB007 MAGAZINE 31 struction. The material can breakdown due to the excessive amount of heat from thermal ex- cursions during all phases of manufacturing, test, and environmental stresses. Do your research and make sure you under- stand, based on your application, whether the material's physical and mechanical properties suit your requirement and the manufacturing process. Fabrication is where the material will probably see its first and worst thermal excur- sions. The amount of weave plays a part in the drilling in some of the HDI laser vias too. Johnson: Are there other specifics in materials that designers should be considering? Creeden: Absolutely. Another major consider- ation nowadays is thermal properties. There are many different market spaces where the end user will place their circuit board in a high-temperature environment. For thermal concerns, we're seeing a lot of people using different polyimide materials, such as automo- tive or military under the hood where the tem- peratures are extremely high. We've talked about the dielectric material, but the other major part of circuit material is the copper. Copper has been our conductive metal of choice going way back. The metal comes in two forms. There's an electrodeposit- ed type of copper both in the holes and on the surface, but there's also a base copper, which is either rolled on to prepreg or clad right onto the core laminate itself. Copper is used because it is highly conductive with low resistivity and will transmit voltage/current. It's affordable, available, and relatively easy to manufacture. The base copper comes in different thickness- es. When you feel it, you're amazed at how easily you can distinguish the weight by hold- ing a couple of sheets of different weight. And copper has an ability for thermal dissipation, which is helpful. If you put a power and ground plane layer next to each other (close so that they couple well) and they're both thick layers of copper, you can typically carry more cur- rent, so that's another property it would have. But when the copper is put onto the prepreg, it needs to bond. When it's laminated, the resin needs to adhere to the metal. Usually, the met- al has a smooth side and a rough side, and the rough side gives you that adhesion required to bond to the material, which will hold it down. And that's measured as a peel strength. This could be realized if you need to do solder re- work, especially if the pads/lands are small, they'll have the potential to peel right off the board, which would destroy it. So, they have this rough side to hold it down. If you look at it in a cross section under a microscope, you'll see how rough it is, which is problematic from a high-speed perspective. And when I explained how the field goes be- tween the bottom side of a trace and the ground plane underneath it, most of the energy is on the bottom side of the trace, which is the rough side. Therefore, the topology of that rough cop- per is not good for what's known as the skin effect where the electrons are bumping on the surface of the copper facing the ground plane. As a solution, the industry has produced very low-profile copper, but it's threatened by that adhesion and peel strength challenge. I'm seeing engineers consider putting the ground plane opposite the smooth side of the copper, trying to guide the wave towards the smooth side. Not all circuits can do that, but that is one way to get around it if you're stuck with the rough side of the copper. Again, getting low- profile copper with good adhesion is the ad- vancement that we need with materials today. In addition, the devices and circuitry are getting smaller and smaller. The standard pin pitch of BGAs are 1.0 mm, but BGA pin pitch keeps getting smaller (0.8, 0.65, 0.5, 0.4, 0.35, 0.25 inches, and smaller). Consequently, high- definition and rectangular, square-edge traces Getting low-profile copper with good adhesion is the advancement that we need with materials today.

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