Printed circuit boards lead-free finishes

Lead-Free Finishes for Printed Circuit Boards and Components... 

The laser has proven to be a versatile tool in many industries. For circuit board assembly, it can be used for marking, single-point soldering, or rework. Lasers can accommodate bonding of the finest or coarsest peripherally leaded surface-mount packages, and its performance and applicabUity are independent of device population density on the board, the thickness of the printed circuit board, or the presence or absence of package heat sinks. It can be used with most any component lead or PWB pad surface finish and any solder, including Pb-free alloys. 

Japanese companies were among the first to market lead-reduced products, typically utilizing a lead-free solder to attach components whose peripheral leads (e.g., Cu, AUoy 42, etc.) were finished with a thin, plated, Pb-free coating to Pb-free features on a printed circuit board (PCB). 

III. LEAD-FREE PRINTED CIRCUIT BOARD FINISHES... 

B. Lead-Free Printed Circuit Board Finishes... 

Lead has been used in var3ring amounts electronic assemblies since their inception. Lead is utilized in the body of some electronic components such as capacitors (lead titanate, lead zirconate), in their interconnections that provide both an electrical pathway and method of attachment to a component carrier, and as a protective finish on the printed circuit boards (PCBs) and component leads. It is also found in peripheral hardware such as batteries, monitors, cables, power supphes, motors, etc. In changing over to lead-free technology, there are several other important considerations aside from just the eUmination of elemental lead (Pb) from components or an assembly. For example, one has to consider the mechanical implications of the alloy replacement... 

There has recently been a renewed interest in Sn whiskers because of the worldwide conversion to Pb-free solders and finishes in electronic manufacturing. Finishes are applied to printed circuit boards (PCBs) and to the lead frames used to connect device packages to printed circuit boards. Lead frames are typically made of a copper (Cu) or iron-nickel (FeNi) alloy plated with a Sn-Pb alloy. Fig. 5 is a schematic diagram of a lead frame cross section bonded to a chip-carrier package. The surface finish of the lead-frame leg is designed to provide surface passivation and enhanced solderability. Typical Pb-free surface finishes are eutectic Sn-Cu or pure Sn. Tin(Sn) whiskers readily grow on high-Sn content finishes under certain conditions. 

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