Electronic Design - The varistor and solder pads are white in the PCBA design

7 special requirements for varistor in PCBA design

Operating temperature

Keep the working temperature of the paste circuit within the working temperature range indicated on the product specification. After mounting, the storage temperature when the circuit is not working is kept within the operating temperature range indicated in the product specification. Do not use at high temperatures exceeding the specified maximum use temperature.

Use voltage

The voltage applied between the terminals of the varistor is kept below the allowable circuit voltage. Misuse will cause product failure and short circuit. The operating voltage is below the rated voltage, but when using it in a circuit where high-frequency voltage or pulse voltage is continuously applied, the reliability of the varistor must be fully studied.

Component heat

Please keep the surface temperature of the varistor below the maximum operating temperature specified in the product specification. The temperature rise of the varistor caused by the use circuit conditions should be confirmed in the actual operating state of the equipment.

working environment

It cannot be used in the following environments. Places with water or salt water; places that are prone to bead and damp; places with corrosive gas; vibration or shock conditions in the use place should not exceed the scope of the product specifications.

Circuit board selection

The performance of the alumina PCB may be deteriorated due to thermal shock (temperature cycling). It is necessary to confirm whether the circuit board has any influence on the product quantity when using it.

Pad size setting

The more the amount of soldering, the greater the pressure that the varistor will experience, and will cause quality problems such as surface cracks on the component. Therefore, when designing the circuit board pad, the appropriate shape and size must be set according to the amount of soldering.

Parts configuration

If the circuit board is bent during the assembly operation of the components, the varistor may be broken. Therefore, the bending strength of the circuit board must be fully considered when configuring the components, and excessive pressure must not be applied.

Reasons for the whitening of PCBA solder pads

1. The reasons for wave soldering are as follows

1. There is thin tin oxide floating on the surface of the wave crest;

2. The preheating temperature or curve parameters are inappropriate;

3. The flux flow rate is too high, the preheating temperature is low, and the tin-eating time is too short;

4. Flux composition, inspection test and certification.

2. The causes after cleaning are as follows

1. Rosin in flux:

Most of the white substances produced after cleaning, storage, and solder joint failure are inherent rosin in the flux.

2. Rosin denatured substance:

This is the substance produced by the reaction between rosin and flux during the soldering process of the board, and the solubility of this substance is generally very poor, it is not easy to be cleaned, and stays on the board to form a white residue.

3. Organic metal salt:

The principle of removing oxides on the soldering surface is that organic acids react with metal oxides to form metal salts that are soluble in liquid rosin, which forms a solid solution with rosin after cooling, and is removed with the rosin during cleaning.

4. Metal inorganic salt:

These may be metal oxides in solder and flux or halogen-containing activators in solder paste, halogen ions in PCB pads, halogen ion residues in the surface coating of components, and halogen-containing materials in FR4 materials that are released at high temperatures. The substances produced by the reaction of halide ions generally have very little solubility in organic solvents. If the cleaning agent is selected appropriately, the flux residues may be removed; once the cleaning agent is not matched with the residues, it may be difficult to remove these metal salts, leaving white spots on the board.

The whitening of the pad after PCBA soldering is mainly caused by the residual flux and not cleanly. It needs to be cleaned after soldering.