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Failure to Start of a Power Supply Using a Linear Regulator, Case 1: Damage to the IC and Peripheral Components Due to Hand-Soldering

2023.11.08

Points of this article

・One possible cause of the failure of a power supply circuit to start (operate) is damage to the IC or peripheral components due to hand-soldering.

・When mounting an IC and peripheral components on a board by hand-soldering, the recommended conditions for hand-soldering the components must be strictly obeyed.

・When components are surface-mounted, or are smaller in size, the possibility of damage due to heat or mechanical stresses during hand-soldering increases.

LDOs and 3-terminal regulators, collectively called linear regulators, are used in a variety of applications due to their low cost and facilitation of simple system designs, and are probably power supply ICs that any designer of electronic circuits has used in the past. However, they sometimes have problems with starting owing to the way they are handled, their conditions of use, and the type of load. In the case of a “Failure to Start of a Power Supply Using a Linear Regulator” considered here, a case is presented in which a power supply circuit using a linear regulator IC fails to start.。

Case 1: Damage to the IC and Peripheral Components Due to Hand-Soldering

Linear regulator ICs and their peripheral components are mounted on PCBs (printed circuit boards) using solder. In mass production, automated equipment regulates the soldering temperature and time involved, so that ordinarily no problems occur. However, when using a soldering iron for hand-soldering, whether to create a prototype, for reworking, or whatever, damage may occur to the IC or to peripheral components, resulting in failure of the power supply circuit to turn on (operate).

Mounting conditions for ICs are described in “Package Information” in “Packaging & Quality” on the relevant website product page (Example: link to ROHM site). Recommended conditions are described for packages for which mounting by hand-soldering using a soldering iron is possible, and soldering operations should strictly conform to these conditions. An example of such conditions is shown below.

＜Example of Recommended Solder Mounting Conditions for a TO252-3 Package＞

・Soldering iron temperature: 380°C or lower
・Mounting time: 4 sec or less (per pin)

However, if these are exceeded, cracks in the package, disconnection of bonding wires, and other problems may occur, and the IC may be damaged. Moreover, when it is necessary to use a soldering iron with a package for which mounting with a soldering iron is not recommended, for example during prototyping, there is increased danger, and so soldering must be performed in a short time, taking extreme care. If the circuit does not start (operate) properly, you should suspect damage to the IC.

Moreover, it is not only ICs that are in danger of being damaged in hand-soldering; there are similar possibilities for surface-mount components that constitute peripheral circuitry, such as resistors and ceramic capacitors. The data sheets for components indicate the recommended conditions for hand-soldering, and these should be strictly observed when performing soldering. Below is an example of the conditions for a peripheral component.

＜Example of Recommended Soldering Conditions for a Resistor ＞

・Iron tip temperature: 350℃
・Soldering time: 4 sec max.
・No. solders: 1
・Power: 20 W max.

*Excluding sizes 0603 (0201), 0602 (01005)

In essence, the smaller the component size, the greater is the possibility of damage to the component when mounting using a soldering iron. In particular, components of sizes 1005 (0402) and below are fragile with respect to external stresses, and excessive heating or pressure from the soldering iron tip may cause separation of the terminal electrode, resulting in an open fault. These issues are especially likely when components are reused. It is difficult to visually judge whether separation of a terminal electrode has occurred. When the output voltage expected from a voltage-dividing resistor does not appear, or when anomalous waveforms are observed, you should suspect damage to a peripheral component.

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