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JANUARY 2026 I I-CONNECT007 MAGAZINE 77 space) of these designs, but it does not have the same thermal conduction as thicker traditional cop- pers. While the planes created in HDI and UHDI are sufficient for transmission line design and EMI/ RFI shielding, they often do not have the same cur- rent density as the thicker coppers. Planes made of thin copper will get significantly hotter than those made of thick copper. Furthermore, these thin coppers are not as toler- ant of surface roughness as traditional thick copper, meaning that the oxide treatment used to improve adhesive bonding/resin adhesion to the plane during the fabrication process cannot be used as extensively. This leads to a potential reduction of bonding strength between the resin and the copper. For example, if standard oxide produces a 2 μm surface roughness to promote adhesion and the overall copper thickness of the layer is 25 μm for 1-ounce copper after processing, it's a reduction of only 8% of the total thickness. On the other hand, the 2 μm surface roughness in a layer of 1/8-ounce Cu, which has an overall thickness of 3 μm, after processing, this 2 μm roughness is now 67% of the overall thickness. Note: These values are for internal copper layers without plating. An 8% reduction in copper thick- ness will have a negligible impact on the electrical performance of the trace. A nearly 70% reduction in copper thickness can have a very significant effect on the electrical performance of the trace. To mitigate this, we specify the use of low profile, very low profile, or ultra low profile coppers. These will maintain the necessary uniform cross-sectional thickness of the trace; however, traditional oxide treatment cannot be used to promote adhesion. On a signal layer, this is not a problem as most of the resin bonding can be from one resin surface to the other resin surface. On a plane layer, however, the majority of the resin bonding is to the copper of the plane. Without the surface roughness, you may run into the fabrication issues I've mentioned. What do designers need to know about proper stackup and plane design? The critical items that the designer needs to know about stackup and plane design include: 1. Regardless of the purpose/function of the board, it is critical to have balanced copper in the Z-axis. This means having the same copper weight, layer type, and layer spacing mirrored above and below the Z-axis centerline. 2. The individual cores also need to be bal- anced with copper. Each core needs to have the same copper weight on both sides of the core. 3. When considering the need for sequen- tial lamination for HDI/UHDI, it is crucial to select materials designed for multiple lam- ination cycles, as well as to understand the changes in material properties result- ing from these cycles. The designer must also understand and account for the impact of these material property changes on the design on planar caps, transmission line design, and other features. What are your dos and don'ts for plane design? Work with the fabricator and assembler to under- stand the overall impact on the design of planes. Consider these EMS vendors' recommendations for improving the design yield. Do not assume that planes can be added without the considerations mentioned above. Treat the planes, especially the planar capacitors made from a pair of adjacent planes, as components in the design, just like any other part in the schematic. Also, remember that while planes and planar capacitors are as close to ideal as you are likely to get, they are not truly ideal. Therefore, remember to consider all the par- asitic elements of all devices in the design. What are the benefits of planes in design? While there has been discussion of several of the challenges in the design of planes and planar capacitors, the benefits of the inclusion of planes in the design far outweigh these risks and chal- lenges. With careful consideration and attention to effects and limitations of planes and planar capac- itors, they will be an effective solution to many of the design challenges related to modern high- speed, fine pitch designs, particularly when used with HDI and UHDI designs. DESIGN007