CHAPTER 4 NOTES ON MOUNTING
4.2 Reflow + Flow Soldering
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In the case of SMD devices, caution is required if reflow + flow mount processing is used. If the temperature of the flow solder bath rises too high, the solder joints of the soldered device will melt again during reflow or become half melted, and the solder joints may peel due to bending, etc., of the device or printed wiring board. Through-holes and connection pins in particular can easily reach very high temperatures due to the heat of the flow solder, and thus caution is required.
Moreover, if the leads of a lead-type SMD are Sn-Pb plated and the solder paste consists of Sn-Ag-Cu, a Sn-Ag-Pb eutectic solder (melting point: 178°C) low-melting-point temperature layer forms due to separation of the Pb, a condition under which peeling can easily occur.
As a countermeasure, it is necessary to lower the solder bath temperature in order to lower the temperature of the solder joint points, or set masking tape or a shielding plate in order to insulate the flow solder and printed wiring board, etc.
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Figure 4-5. Peeling Mechanism in Case of SMD Device (BGA)
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| Figure 4-6. |
Example of Solder Joint on Device Side That Broke During Flow Soldering after Reflow |
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