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PCBA Tech
Improve tin penetration and the role of chip capacitors in PCB
PCBA Tech
Improve tin penetration and the role of chip capacitors in PCB

Improve tin penetration and the role of chip capacitors in PCB

2021-11-10
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Author:Downs

PCBA production process to improve tin permeability

Regarding PCBA tin penetration, we should understand these two points:

 1. PCBA tin penetration requirements

According to the IPC standard, the PCBA tin penetration requirement of through-hole solder joints is generally more than 75%. That is to say, the tin penetration standard for the appearance inspection of the panel surface is not less than 75% of the hole height (board thickness). PCBA The tin penetration is suitable at 75%-100%. The plated through hole is connected to the heat dissipation layer or the heat conduction layer for heat dissipation, and the PCBA tin penetration requires more than 50%.

2. Factors affecting PCBA tin penetration

The poor tin penetration of PCBA is mainly affected by factors such as material, wave soldering process, flux, and manual soldering.

Specific analysis of factors affecting PCBA tin penetration:

1. Material

Tin melted at high temperature has strong permeability, but not all metals to be welded (PCB boards, components) can penetrate in, such as aluminum metal, whose surface generally automatically forms a dense protective layer, and the internal molecules The difference in structure also makes it difficult for other molecules to penetrate. Second, if there is an oxide layer on the surface of the metal to be welded, it will also prevent the penetration of molecules. We generally use flux to treat it or brush it with gauze.

pcb board

2. Wave soldering process

PCBA tin penetration is directly related to the wave soldering process. Re-optimize the welding parameters with bad tin penetration, such as wave height, temperature, welding time or moving speed. First, reduce the orbital angle appropriately and increase the height of the wave crest to increase the contact amount of liquid tin with the soldering end; then, increase the temperature of wave soldering. Generally speaking, the higher the temperature, the stronger the permeability of tin, but this should be considered. The components can withstand the temperature; finally, the speed of the conveyor belt can be reduced, and the preheating and soldering time can be increased, so that the flux can fully remove oxides, infiltrate the soldering ends, and increase the amount of tin consumed.

3. Flux

Flux is also an important factor affecting PCBA's poor tin penetration. Flux mainly plays a role in removing surface oxides on PCB and components and preventing re-oxidation during soldering. Flux selection is not good, coating unevenly, and the amount is too small. Will lead to poor tin penetration. A well-known brand of flux can be selected, which will have higher activation and wetting effects, and can effectively remove difficult-to-remove oxides; check the flux nozzles, and the damaged nozzles need to be replaced in time to ensure that the PCB surface is coated with a proper amount of flux. Give full play to the flux effect of the flux.

4. Manual welding

In the actual plug-in welding quality inspection, a considerable part of the weldment only has a taper on the surface of the solder, and there is no tin penetration in the via. The function test confirms that many of these parts are soldered. This situation is more common in manual plug-ins. During soldering, the reason is that the soldering iron temperature is not appropriate and the soldering time is too short. Poor PCBA tin penetration can easily lead to false soldering problems and increase the cost of rework. If the requirements for PCBA tin penetration are relatively high, and the soldering quality requirements are relatively strict, selective wave soldering can be used, which can effectively reduce the problem of poor PCBA tin penetration.

The role of chip capacitors on PCB circuit boards

SMD capacitor is a kind of capacitor material. SMD capacitors are called: multilayer (multilayer, laminated) chip ceramic capacitors, also known as SMD capacitors, chip capacitors. There are two ways to express the chip capacitor, one is expressed in inches, and the other is expressed in millimeters. SMD capacitors mainly have the following functions on the PCB circuit board.

1. Bypass

The bypass capacitor is an energy storage device that provides energy for the local device. It can uniformize the output of the regulator and reduce the load demand. Just like a small rechargeable battery, the bypass capacitor can be charged and discharged to the device. In order to minimize the impedance, the bypass capacitor should be as close as possible to the power supply pin and ground pin of the load device. This can well prevent the ground potential rise and noise caused by the input value being too large. The ground potential is the voltage drop at the ground connection when a large current glitch passes through it.

2. Decoupling

Decoupling, also known as decoupling. From the circuit point of view, it can always be divided into the driving source and the driven load. If the load capacitance is relatively large, the drive circuit must charge and discharge the capacitance to complete the signal jump. When the rising edge is relatively steep, the current is relatively large, so that the drive current will absorb a large power supply current. The inductance and resistance (especially the inductance on the chip pins will bounce). Compared with the normal situation, this current is actually a kind of noise, which will affect the normal operation of the previous stage. This is the so-called "coupling" .

The decoupling capacitor acts as a "battery" to meet the change of the drive circuit current and avoid mutual coupling interference.

Combining bypass capacitors and decoupling capacitors will make it easier to understand. The bypass capacitor is actually decoupled, but the bypass capacitor generally refers to high-frequency bypass, that is, to improve a low-impedance leakage prevention method for high-frequency switching noise. High-frequency bypass capacitors are generally relatively small, generally 0.1μF, 0.01μF, etc. according to the resonance frequency; while the capacity of decoupling capacitors is generally larger, which may be 10μF or greater, depending on the distribution parameters in the circuit and the change in drive current to make sure. Bypass is to take the interference in the input signal as the filtering object, and decoupling is to take the interference of the output signal as the filtering object to prevent the interference signal from returning to the power supply. This should be their essential difference.

3. Filter

Theoretically (that is, assuming that the capacitor is a pure capacitor), the larger the capacitance, the smaller the impedance, and the higher the passing frequency. But in fact, most of the capacitors over 1μF are electrolytic capacitors, which have a large inductance component, so the impedance will increase when the frequency is high. Sometimes you will see a large electrolytic capacitor with a small capacitor connected in parallel. At this time, the large capacitor is connected to the low frequency and the small capacitor is connected to the high frequency. The function of the capacitor is to pass high impedance and low impedance, and pass high frequency to block low frequency. The larger the capacitance, the easier it is to pass low frequencies. Specifically used in filtering, a large capacitor (1000μF) filters low frequencies, and a small capacitor (20pF) filters high frequencies. Some netizens have vividly compared the filter capacitor to a "pond." Since the voltage at both ends of the capacitor does not change suddenly, it can be seen that the higher the signal frequency, the greater the attenuation. It can be said that the capacitor is like a pond and will not change the amount of water due to the addition or evaporation of a few drops of water. It converts changes in voltage into changes in current. The higher the frequency, the greater the peak current, thus buffering the voltage. Filtering is the process of charging and discharging.

4. Energy storage

The energy storage capacitor collects the charge through the rectifier and transfers the stored energy to the output terminal of the power supply through the lead of the converter. Aluminum electrolytic capacitors with a voltage rating of 40~450VDC and a capacitance value of 220~150 000μF (such as B43504 or B43505 of EPCOS) are more commonly used. According to different power requirements, PCB devices are sometimes used in series, parallel, or a combination of them. For power supplies with a power level of more than 10KW, larger tank-shaped screw terminal capacitors are usually used.