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PCB007-Oct2020

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52 PCB007 MAGAZINE I OCTOBER 2020 the circuitry that carries the current through- out the board. Parts coming to the electroplat- ing line have already been through the elec- troless copper process. Pretreatment here in- volves a cleaning step, micro-etch, and acid predip. The "cleaner" serves a series of functions: the detergent component removes soils and organ- ic residues (fingerprints), the acidic component removes oxidation, and the surfactant present wets the surface. A properly wetted surface will help dislodge any entrapped air in the narrower vias. Entrapped air leads to voiding and discon- tinuity. Vibrating the parts in the cleaner bath is recommended for high aspect ratio holes and blind vias. Vendor recommendations of make- up, bath life, operating temperature, and dwell time, should be followed. Good rinsing should follow the "cleaner." The micro-etch step is mild and removes 5–10 micro-inches of copper, exposing a fresh surface for plating. Excessive etching is not recommended, as it may dissolve the electro- less metallization resulting in voiding. The last step in pretreatment is an acid pre- dip. This serves two functions. First, it ensures no oxidation on the copper surface; second, it acidifies the surface to match the acidity of the copper electrolyte. Surface Finish For the purpose of this column, I chose to break down surface finish to: 1. Single-layer finishes: Hot air solder leveling (HASL), organic solderability preservative (OSP), immersion silver, and immersion tin 2. Multilayer finishes: ENIG, ENEPIG, and EPIG/EPAG Single-Layer Finishes The pretreatment for this class of finishes is based on creating a clean surface that is con- taminant/oxide free. In addition, for immersion silver and immersion tin, the surface morphol- ogy of the copper surface is also addressed. For HASL to produce a contiguous inter- metallic (IMC) with no dewetting, the copper The process is referred to as "pickling" and usually involves the use of strong acids like sulfuric or hydrochloric. The pickling liquor may contain other acids as well as specific ad- ditives like wetting agents and corrosion inhib- itors. The pickling chemistry must be compat- ible with the specific basis metal and effective at removing the associated impurities and or contaminants. The choice of pickling liquor chemistry (type, concentration, and bath life) and the condi- tions under which it is used—like tempera- ture, dwell time, and part agitation—must be well-defined and adhered to for efficient sur- face preparation. The pickling solution must be thoroughly rinsed before the introduction into the plating electrolyte. 2. Plating for Electronics Manufacturing Electroless Copper The electroless copper process is used in electronic manufacturing to metalize the di- electric in the drilled holes. The deposition is autocatalytic or electroless and does not re- quire rectification. Pretreatment is very critical in this process as it serves multiple functions. It starts with a cleaner/conditioner that cleans the surface and modifies the fiberglass in the FR-4 dielec- tric by altering its negative charge to neutrality so that it will adsorb the catalyst that initiates the copper deposition. The cleaner is followed by a micro-etch that refreshes the copper on the surface, as well as in the inner layer interconnects in the hole. It en- sures good adhesion on the copper surface and the interconnect integrity with no separation be- tween the plated copper and the inner layer. This step is followed by the catalyst and its rinse. If the pretreatment is compromised, voiding in the deposit may occur as well as a sepa- ration in the interconnect at the inner layers. The electroless copper process is preceded by "desmear." Desmear is considered a process in its own right and not a pretreatment. Acid Copper Electroplate The copper electroplate process is the back- bone of printed circuit manufacturing. It forms

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