| Summary: | 碩士 === 國立高雄應用科技大學 === 化學工程與材料工程系碩士在職專班 === 103 === Passive components are mounting and soldered on the PCB via Surface mounting technology (SMT) and the solder joints will be checked by automated optical inspection (AOI). Wetting conditions and height of the solder determine the reliable solder joint between the component and the PCB. During reflow soldering, the oven temperature is stabilized at 260 ℃, and the PCB will be sustained for about 40~60 sec at this temperature. At 260 ℃, the solder paste on the PCB will melt and form solder joint with the tin layer of the passive components.
When the passive components are used in automotive, LED and other applications with severe thermal cycling, solder joints of the components are often destroyed resulting in these components cannot meet the specified life time, digital X-ray inspection are then used to evaluate the solder joints of the SMT components. Through X-ray diffraction technique (XRD inspection), it was observed around 10~50 % voids in the solder joints which passed the AOI inspection with good wetting and over 2/3 solder height. Therefore, the size and area of voids in solder joints may become an important life indicator of SMT components in automotive and other important applications.
This paper study to modify the characteristics of plating layer of the components to address the elimination of voids and further investigate its causes. The plating layers of passive components, the resistor as an example, is nickel plated and finally tin plated, the thickness of the nickel plating is about 4~10 μm and the thickness of the tin plating is within 4~12 μm.
Nickel plating generally prevents the silver layer (sintered layer) diffusion into the tin layer and allows the tin layer to maintain a good soldering thickness. The tin plating provides the medium for wetting with the solder paste and is the most important factor for the component solderability. This paper investigates the effect of tin plating layer on the solderability and voids in the solder joints.
This paper investigates the characteristics changes of the tin plating layer as follows:
1. Grain size
2. Plating layer thickness
3. Post treatment
4. Sn, Sn-Bi, Sn-Cu, Sn-Ag alloys
and uses a 0603 size resistor to study the voids distribution in the solder joints via XRD inspection after SMT reflow soldering.
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