Solder connection, electronic devices

Recently commercially available X-ray systems for laminography have a spatial resolution limited to hundred microns, which is not enough for modem multilayer electronic devices and assembles. Modem PCBs, flip-chips, BGA-connections etc. can contain contacts and soldering points of 10 to 20 microns. The classical approach for industrial laminography in electronic applications is shown in Fig.2. 

When electronic products are constructed, individual components such as chips, condensers, or conductor boards must be connected to fully functional electronic devices. This crucial step is usually performed using reflow or wave soldering processes. Both processes can be performed either in air or in controlled and inert atmospheres (see Fig. 2.24). Using a controlled atmosphere, however, provides the following substantial benefits ... 

The conventional method using screen printed solder pastes or isotropic conductive adhesives is drflicult to apply when the distance between adjacent connections is less than 0.5 mm. Thus, the main impetus for developing anisotropic conductive adhesives has been the fine pitch interconnection of electronic devices... 

In addition to the sensors, other electronic elements are necessary for pregnancy monitoring. These include low-power amplifiers, processing units, and radios to transmit the data to a personal device. Nowadays, these are stiU hard elements soldered to printed circuit boards (PCBs), which can be sewn into the garment. An example is the Lilypad Arduino (Arduino, 2007), which is a microcontroller board designed especially for use with wearables. The board connection pins are holes that can be sewn... 

The development of surface-mounted technology (SMT) using surface-mounted devices (SMD) was a major development. SMD are active and passive electronics components without conventional connecting wires. SMD components can be placed on the solder side of the PCB and their metal caps soldered to the copper pads of the PCB. Both layers of the PCB can therefore be used as active areas. The SMT technique opened advantages and new applications through miniaturisation of components and increased reliability. 

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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