Issue link: https://iconnect007.uberflip.com/i/1248324
82 PCB007 MAGAZINE I MAY 2020 is carried out in four steps. PTFE means polytet- rafluoroethylene (a common trade name is Teflon®; it belongs together with perfluorethyl- enpropylene (FEP), perfluoralcoxipolymers (PFA), and ethylene-tetrafluroethylene-copolymer (ET- FE) to the fluoropolymers. All fluoropolymers can be pretreated with plasma in a similar way. These products are available as pure blends or with fillers. The fillers generally improve the properties of the laminate, thermal conductivity, coefficient of thermal expansion, electrical properties, and mechanical properties. They can make pro- cessing easier. The types of fillers are generally not public; they can be talc, silica, hollow glass spheres, or ceramics. Even PTFE can be used as filler. Plasma activation of PTFE and pure blends can be done in one step with ammonia (NH 3 ) or with a mixture of hydrogen and ni- trogen. It is possible to perform the process in three steps. When PTFE with fillers (ceramic, glass) are used, two segments are required. The filler material has to be pre-etched before the PTFE is activated. To do this, CF 4 /O 2 is introduced in the first segment, followed by the second seg- ment of H 2 /N 2 to activate the PTFE. The process in one step is as follows: 1. Ammonia or hydrogen and nitrogen (70%H 2 /30%N 2 , 10–30 minutes). This process in three steps is as follows: 1. Heat up oxygen with 10–20% of nitrogen to reach 70–90°C. 2. Surface etch 80% hydrogen with 20% nitrogen (30 minutes). 3. O 2 burn, 100% O 2 , to remove residues (5 minutes). PTFE Together With Other Types of Laminate [3] Multilayer PCBs consist of several different materials to reach thermal and signal require- ments. Different types of prepregs are lami- nated together and give a multilayer. There are many possible combinations of different types of epoxy, polyimide, and fluoropolymers. Desmearing panels with mixed material can also be done through plasma. Laminated com - binations, such as epoxy and PTFE, require desmearing and PTFE activation. Because hy- brid panels are partially made up of the same resins as standard rigid panels, the drill smear must still be removed. PTFE does not smear. However, it must be activated in the hole wall to increase wettability. Desmearing and PTFE activation in the same process uses four gases and four steps. This allows hybrid panels to be processed for electroless plating in one step. No glass fiber removal is required for this process. This process in four steps is as follows: 1. Oxygen until 80–90°C is reached on the board surface (oxygen and 10% inert gas, nitrogen/argon). 2. Mixture of CF 4 and oxygen with 10% inert gas (10–30 minutes). 3. Oxygen with 10% inert gas to burn the residues (5–10 minutes). 4. Mixture of hydrogen and nitrogen (3:1) or ammonia (5–20 minutes). This process in two steps is as follows: 1. Mixture of CF 4 and O 2 with 10% of inert gas, nitrogen or argon (5–30 minutes, depending on the energy of the RF generator). 2. Mixture of H 2 and N 2 (3:1) or ammonia (5–20 minutes). Surface Cleaning Before Solder Mask [3] Cleaning the surface before solder mask makes sense if copper deoxidation of the sur- face is needed to prepare the laminate before solder mask. The first step is chemical preclean and microetch. Next, follows plasma cleaning in one step with an inert gas like argon. It is possible to perform this plasma step with other gases, too, such as hydrogen or helium. Note, however, that if the outer layer is made of PTFE, you will want to proceed as described in the section on PTFE. Surface Treatment of PCBs: Increasing Surface Tension [3] Plasma treatment can also be used to im- prove solder mask surface adhesion for leg-