PCB007 Magazine

PCB-May2015

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48 The PCB Magazine • May 2015 cessing. The cleaner removes oxides and light surface contaminants, and ensures that the surface will be in a condition allowing it to be uniformly micro-etched. Vendor specifications for temperature, dwell time, agita- tion and bath chemical control should be followed. 2. Micro-etch: This step produces a surface that may be uniformly catalyzed and plated with good deposit ad- hesion by removing some copper from the surface. A variety of different etchant types may be used (e.g., sodi- um persulfate, peroxide/sul- furic). Vendor specifications for temperature, dwell time, agitation and bath chemical control should be followed. 3. Catalyst: The step de- posits a material that is cata- lytic to electroless nickel plat- ing on the copper surface. The catalyst lowers the activation en- ergy for nickel deposition and allows plating to initiate on the copper surface. Ex- amples of metal catalysts include palladium and ruthenium (deposited by an immersion reaction with the copper surface). Vendor specifications for temperature, dwell time, ag- itation and bath chemical control should be followed. 4. Electroless Nickel: The purpose of this bath is to deposit the required thickness of electroless nickel on the catalyzed copper surface. The nickel thickness should be ade - quate to cover the copper with a substantially pore-free coating, to create a diffusion barrier to copper migration, and also serve as a sol- derable surface, depending on the intended application. The nickel bath has a relatively high depo- sition rate and its active chemical components must be maintained in balance on a continuous basis, by addition of appropriate replenishment components. Electroless nickel baths typically run at high temperatures and extended dwell times to achieve the required deposit thickness. It is therefore important to ensure that compat- ible PWB substrate and solder mask materials are used. Vendor specifications for temperature, dwell time, agita- tion and bath chemical control should be followed. 5. Electroless Palladi- um: This bath deposits the desired thickness of electro- less palladium on the freshly deposited nickel surface. It is mainly composed of a source of palladium metal ion and a reducing agent. In addition, it has proprietary stabilizers, sur- factants and chelating agents. The solution requires replen- ishment of all of its compo- nents to maintain its activity. Vendor specifications for tem- perature, bath life, dwell time, agitation and bath chemical control should be adhered to. 6. Immersion Gold: The purpose of this step is to deposit a thin, continuous layer of immersion gold. This is a displacement reaction. Note: As a result of their respective position in the electro- motive series, the exchange rate of gold with the palladium substrate is not as efficient as gold with the nickel substrate. The gold protects the underlying electroless palladium layer from reacting with the ambient environment and also serves as a contact sur- face, depending on the intended application. This bath runs at relatively high temperatures and dwell times. Vendor specifications for tem- perature, dwell time, agitation, bath life and bath chemical control should be followed. the catalyst lowers the activation energy for nickel deposition and allows plating to initiate on the copper surface. Examples of metal catalysts include palladium and ruthenium (deposited by an immersion reaction with the copper surface) " " FEaturE ENEPIG: THE PLATING PrOCESS continues

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