Issue link: https://iconnect007.uberflip.com/i/1527613
12 DESIGN007 MAGAZINE I OCTOBER 2024 smaller, you are not able to place a mechani- cally drilled plated through-hole in there and still meet the IPC design and producibility requirements. You also need to go to very thin coppers to etch the feature geometries that small. We call this partial HDI, but many com- panies call it hybrid construction. Fortunately, we can now implement some HDI technol- ogy in one lamination cycle without going through full sequential lamination, which is a big advantage. Shaughnessy: I imagine that there are some trade-offs when we mix technologies like this. Yes. e problem is, it's not just that the pitch is smaller; the size of the land is also smaller, so the spacing between the lands is smaller. In traditional boards, we know how to work with half-ounce copper, or 1-ounce copper. But now we're in quarter-ounce, eighth-ounce, 20-micron, 15-micron, or 5-micron copper thicknesses to get the etching that fine. With a 0.5 millimeter pitch BGA, we're talking about 3 or maybe 2 mils of copper-to-copper between the lands. You can't etch that out of traditional 1-ounce copper. With these hybrid designs, you can build up all the inner layers with standard 1-ounce copper, but on the outer layer where these BGAs are being attached, you will have to go to very thin coppers and doing some laser- drill microvia technology to get a via feature sufficiently small enough that gets between the pads to get down to that first layer from the sol- der pads. Marcy LaRont: You don't have to do 2-4-2 buildup boards? Is that what we're talking about here? at's right. e 2-4-2 is full HDI with sequen- tial lamination. e number 4 in 2-4-2 means that you build four layers using standard mul- tilayer fabrication. en on each of the two sides, the top and the bottom, you add two HDI layers sequentially. With partial HDI, we only need one HDI layer on that one surface. You can build it in one lamination cycle because you only have to laser drill that first outermost layer. But you will need a thin layer to meet the aspect ratio requirements of microvias. You might not necessarily have a reinforcement layer. at's one of the issues that we have with HDI. Oen, to get those very thin dielectrics— the 1-mil dielectric layers that we do with HDI layers—we're basically doing resin-coated cop- pers without any reinforcement material. We have very thin dielectrics and coppers on the outermost layer pair—which become our top and bottom—and this is where the prob- lems come in. We call it hybrid or partial HDI because we're not going all the way to sequen- tial lamination. Now, we don't need to use specialty laminate materials that are approved by the suppliers for multiple laminations and designed to survive multiple lamination cycles. Multiple lamination cycles tend to dry out the resins each time they go through a cycle, and that causes a shi in some of the properties of the layers and so on. at's why many materials are currently defined as, "Good for up to five lamination cycles." Kris Moyer