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PCBD-Apr2015

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14 The PCB Design Magazine • April 2015 spec could be lifted, to limit the typical pad size to be measured and to make the specification applicable to LF solder and LF conditioning. This entails an extensive RR study, presently in progress. The intent is that the revised IPC-4552 Rev A would also include the following Additional Documents: • Test method (TM) for stripping immersion gold during failure analysis • Test method for determining the phos content of electroless nickel • A corrosion chart setting acceptability criteria for nickel corrosion (black pad) Immersion Silver (IAg) IPC-4553 A, 2009 IAg is a thin, immersion silver deposit over copper. It is a multifunctional copla- nar surface finish, applicable to soldering. It may also be applicable for some press-fit connections and as a contact surface. It has the potential to be suitable for aluminum wire bonding. The immer- sion silver protects the un- derlying copper from oxida- tion over its intended shelf life. Exposure to moisture and air contaminants, such as sulfur and chlorine, may negatively impact the useful life of the deposit. The im- pact can range from a slight discoloration of the deposit to the pads turning completely black. Proper packaging is a requirement to achieve a 12-month shelf life. The Immersion Silver IPC-4553 Specification In 2005, there were two distinct types of commercialized immersion silver with differ- ent thickness recommendations, referred to by the committee as "thin" and "thick." Each re- quired its own thickness specification. This cre- ated much confusion as the terms were poorly defined. The initial IPC-4553 Immersion Silver Speci- fication specified two thicknesses and stated the following for thickness of deposit: • Thin Silver: 0.05 µm(2µin) minimum at -2σ from process mean as measured on a pad of area 2.25² µm (3600² mils). Typical value 0.07 µm (3µin) to 0.1 2µm (5µin) • Thick Silver: 0.12 µm (5µin) minimum at -4σ from process mean as measured on a pad of area 2.25² µm (3600² mils). Typical value of 0.2 µm (8µin) to 0.3 µm (12µin). In 2009, the immersion silver specification was revised. At this time the lower thickness supplier has discontinued his product and the industry was left with a common thickness from multiple suppli- ers. This revised specification only had one thickness speci- fied (eliminating reference to thin and thick). The revised specification now includes an upper thickness limit. Typi- cal values were recommended within the specified limits. The pad size for taking the thickness measurement was also specified. The IPC-4553 Rev A Im- mersion Silver Specification stated the following for thick- ness of deposit: • The immersion silver thickness shall be 0.12 µin [5 µin] minimum to 0.4 µm [16 µin] maximum at ± 4σ from process mean. Typical value between 0.2 µm [8 µin] to 0.3 µm [12 µin] as measured on a pad of area 2.25 mm² or 1.5 mm X 1.5 mm [approximately 0.0036 in² or 0.060 in X 0.060 in] or equivalent. Immersion Tin (ISn) IPC-4554, 2007; amended 2012 The immersion tin (ISn) is a metallic finish deposited by a chemical displacement reaction that is applied directly to the basis metal of the printed board, which is copper. The immersion IPC SURFACE FINISH SPECS UPDATE continues feature In 2009, the immersion silver specification was revised. at this time the lower thickness supplier has discontinued his product and the industry was left with a common thickness from multiple suppliers. this revised specification only had one thickness specified (eliminating reference to thin and thick). " "

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