Issue link: https://iconnect007.uberflip.com/i/1522641
18 PCB007 MAGAZINE I JUNE 2024 unreinforced dielectric material. e indus- try vernacular for this is bondply. It is a non- glass reinforced resin that can be delivered in two standard formats: resin-coated copper foil (RCC) or resin-coated film (RCF). ere are many ways of delivering resin into a circuit board. e standard way is to use pre- preg, which is a resin-impregnated fiber, and usually a glass fabric. Another option is to screen-print the resin onto the printed cir- cuit board material, which has been available for years. en there is the bondply technique, which has a few specific advantages. We heard about this at a recent UHDI conference. Please explain bondply. It's a dielectric layer used in multilayer stackups where you want to build bonding in sequential layers. Imagine you have a circuit, and you want to make HDI layers on the outside with very thin dielectrics, 25–50 microns, just one or two mils. You take the board, which has copper on one side and laminate a layer of bondply onto it. In this case, that could be resin-coated cop- per foil. You form a pattern on the copper and laser drill through that layer to connect down to the layer beneath. You can build up layers that way, adding one layer aer another. is is a standard technique for making multilayer, high density circuits, typically with microvias on the outside using such standard construc- tions as 2+4+2 or 2+8+2, etc. at is one area where these bondply films can be used and it is a standard method of forming thin layers. Is it technically a film? Yes, we call it resin-coated film (RCF). RCF and RCC are the two terms you'll generally hear, and this film can be used on different types of materials. e main thing is we have a dielectric here that's not glass reinforced. Why non-glass? ere are several rea- sons. First, you can get thinner layers; sub-25 microns is a challenge with glass fibers. e thinnest glass filaments we use are around 5 microns. So, you only have to get four or five fibers laid together in a bunch to reach 25 microns. For thin layers, bondply technology is important. Second, it helps with signal integrity. e glass and resin of standard materials form the composite material that we call the substrate, and those two things have very different electri- cal properties. Typically, the resin is quite lossy but has a fairly low dielectric constant (Dk). It doesn't store much charge. Glass is the other way around, with a relatively high dielectric constant and low loss factor. You always have to balance between the two materials. As a sig- nal passes over a glass fiber, you get impedance changes and that means velocity of transmis- sion speed changes. e speed of your trans- mission line changes as you pass over both cop- per-rich and resin-rich areas. If you just do a resin layer alone, you can make a homogeneous layer that doesn't have these differences. is is one of the most significant benefits. Interposers are another reason to use bond- ply material because you want to add some kind of pliability. Rubber, for example, is a mate- rial with a low modulus, so it can be pliable. Alun Morgan