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Creation QR Code in Software PRINTED CIRCUIT BOARD SURFACE FINISHES

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copper. The intermetallics formed between nickel and tin are very thin and more physically brittle than the copper tin Intermetallic Compound (IMCs). Electroless palladium can also act as a barrier layer if used in sufficient thickness. Automotive PCBs produced in the 1990s specified a maximum of 0.2 micron of Pd metal with the goal to incorporate the entire Pd layer into the solderjoint. Figure 32.5 shows the interaction of four surface finishes with solder during the assembly process.
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Flux
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CuSn intermetallic forms at each temperature step HASL
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Cu = 4.1 u"/sec OSP
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Au A = 117.9 u"/sec Ni = 0.05 u"/sec ENIG
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Ag = 43.6 u"/sec Ag or Sn
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FIGURE 32.5 Displacement of PCB surface finishes during component assembly and soldering. (a) HASL: PCBs show some intermetallic formed during the HASL operation. Upon subsequent assembly, the HASL coating melts into the solderpaste or wave solder, and the intermetallic grows in thickness. (b) OSP: The OSP coating is removed by the flux during component assembly. The underlying copper forms the intermetallic with tin during soldering. (c) ENIG: Gold dissolves quickly into the tin phase of solderpaste or wave solder. The intermetallic is formed between tin and nickel because the rate of dissolution into nickel is slow. (d) Silver or Tin: Silver and tin coatings dissolve very quickly into the tin phase of solder. The intermetallic is formed with underlying copper.
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RELIABILITY TEST METHODS
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Reliability is determined using a variety of methods, usually used in combination. Often an unassembled PCB is exposed to a conditioning or aging environment. The conditioning may consist of time, temperature, humidity, and contamination metrics. The conditioning is normally at a higher level than the environment in which the device is intended to be used so that the test environment imparts an accelerated age condition. Temperature exposure for PCBs is commonly conducted at 155 C for 4 or 8 hours to study shelf-life. To study the effect of soldering, simulation is conducted at the actual reflow temperature, without contact with flux or solder. Temperature cycling of assembled devices reflects the difficulty of the intended end-use environment.Although some cycling may occur at 0 C to 100 C, more severe cycling may require 55 C to 125 C. Assembled parts may also undergo physical testing conditions such as physical shock, vibration, bending, twisting, and impact. Environmental exposure testing is common for PCB surface finish evaluation. Exposure to ambient air during reflow is common, as is long-term exposure to polluting environments such
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as mixed flowing gas. Physical contact with contaminating materials such as fluxes, assembly materials, fuels, and even diesel fuel and carbonated beverages has been documented. Electrical testing is another class of reliability testing. The function of a solderjoint formed with the surface finish can be measured by burn-in testing, electrical functional tests, and/or high-current-density application for extended periods. The contact resistance of an unassembled surface finish is an important measurement. The PCB finish, when exposed to heat and humidity metrics, as well as environmental exposure, may need to pass a sensitive four-point probe contact resistance check.
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32.11.1 High-Speed Signals Electrons travel along the outer regions of a conductor. This phenomenon, known as the skin effect, is most pronounced at the high signal frequencies of Global Positioning Systems (GPS), aerospace, navigation, mobile telephony, computer servers, and crash avoidance devices. At high frequencies, the skin depth of electron travel is of a thickness similar to that of the PCB surface finish. For example, at 10 GHz, the skin depth is less than 1 micron. Therefore, the electrical conductivity of surface finish materials is important at those higher frequencies. Above 2 GHz, circuit designers find that the use of electroless nickel hinders signal integrity. Silver, the most conductive element, is used for devices operating at higher frequencies.
32.11.2 Inspectability To ensure good soldering, it is necessary for the assembler to ensure that all copper is protected and no solder mask residue remains on solderable features. By inspecting the bare board, the engineer can reject parts that show bare copper due to skipped surface finishing or mask residue. A special case is OSP, where inspection of the protected copper is impossible because OSP-protected areas have the same visual appearance of uncoated copper with or without thin solder mask contamination.
32.11.3 Contact Functionality Many circuit board assemblies rely on surface contact functionality to operate. Examples include touchpads, edge contact rails for grounding/shielding, test probe contact points, zero insertion force connectors, chassis and enclosure bolts, and surface contacts for mating with nonsoldered connectors/interposers. OSP does not meet the demands of surface contact, so selective plating or soldered contact designs must be used. Immersion tin may lose contact functionality as the intermetallic oxidizes. HASL will maintain contact ability, but suffers from material creep under load, which may allow loose connections after field use. Silver maintains contact functionality even when tarnished, but loses contact ability if heavily corroded by the environment. ENIG maintains contact functionality over long periods of time, but is also susceptible to excessive corrosion. Electroplated Ni/Au is the best performing finish for long-term contact functionality.
32.11.4 Wirebonding Some device designs call for wirebonding directly to the PCB or laminate chip carrier. In general, most surfaces are wirebondable with aluminum wire. Al wirebonding may be impossible
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