Issue link: https://iconnect007.uberflip.com/i/1475604
50 DESIGN007 MAGAZINE I AUGUST 2022 PCB fabricators are aware of these issues. When a multilayer hybrid PCB is made by combining FR-4 and a PTFE-based material, through-hole drilling can be a major concern. e PTFE material is very so, and the FR-4 material is rigid, and that interface can be a concern for drilling the through-holes. e drill tool and parameters are critical to ensure a good quality hole is drilled without overheat- ing the PTFE material. If the PTFE material is overheated, it can cause "flap" at the FR-4 and PTFE interface. e flap is a small amount of PTFE that has been smeared over the metal interconnect and that can be a reliability con- cern. Unfortunately, if PTFE is smeared and a flap exists, there is nothing that can desmear PTFE and the circuit may be discarded. Another circuit fabrication issue for hybrids is the treatment of the through-hole prior to electroless copper plating. From the above example, the PTFE material will need a differ- ent process to make it wettable as compared to the FR-4 material. Generally, the FR-4 material can go through a plasma or permanganate pro- cess to get the material wettable, which allows it to accept the electroless copper plating in the drilled hole. However, the PTFE material will usually require a very different process and the circuit fabricator should know the correct pro- cesses, the sequence to run these processes, and any post processing that may be necessary. If the circuit fabricator is unaware of these pro- cesses, our technical service engineers can get involved and assist with the process definition. ere can be benefits to combining dissimi- lar materials to help reliability issues. If one of the layers in the hybrid multilayer circuit has a poor CTE, it is possible to combine dissimilar materials with good CTE to get an overall good CTE for the circuit. In many cases this can help assembly concerns and long-term reliability. However, there can be exceptions, and this will need to be well understood, because it is still possible to get poor reliability performance on the layer with the high CTE property. ermal management can also benefit from the use of dissimilar materials. If a hybrid PCB has one layer with poor thermal conductiv- ity and other layers with very good thermal conductivity, that can help the overall ther- mal performance. Again, there are exceptions, and this will need to be well understood by the designer and circuit fabricator to get the improved thermal performance. ere are also hybrids that combine dissimi- lar high frequency circuit materials for custom- ized electrical performance. One concept that has been used many times is to use a broad- side coupled stripline circuit with the material between the signal planes having a high Dk, and the materials between the ground-signal planes having a lower Dk. e lower-Dk mate- rials will allow a wider signal conductor and that will give lower conductor losses. High Dk material is very good at condensing electric fields, so having the high-Dk material between the coupled signal planes will increase the cou- pling. is is one way to get a tightly coupled circuit with lower losses. Hybrid multilayer PCBs have been used for many years and the trend will continue. It is a way to get more performance out of a circuit, sometimes improving reliability and possibly reducing the cost of the circuit. DESIGN007 John Coonrod is technical marketing manager at Rogers Corporation. To read past columns, click here. Combining low-cost FR-4 materials with higher quality high-frequency circuit materials is often done with today's hybrid PCBs.