During circuit board soldering process, what should be paid attention to?
1. Possible conditions to drop the conversion rate of low signal edges
Meeting the design criteria while trying to choose slower devices, usually at the time of device selection. And to prevent the mixed using of different types of signals, because the fast transformed signal has the potential crosstalk risk to the slowly transformed signal.
2, Using shielding measures
Grounded will incur an increase in the amount of wiring, and providing grounded for high-speed signals on the circuit board is an effective way to deal with crosstalk problems. But making the already limited wiring area more congested. In addition, it is critical that the ground shield is to reach the grounding point spacing on the intended destination line, which is typically less than twice the length of the signal change edge. At the same time, the ground line also increases the spreading capacitance of the signal, so that the impedance of the transmission line increases and the signal edge becomes slower.
3, Reasonable setting of layers and wiring
Reducing the parallel signal length and setting the wiring layer and wiring spacing reasonably. Shortening the spacing between the signal layer and the planar layer, increasing the signal line spacing, and reducing the length of the parallel signal lines (within the critical length range) can effectively reduce crosstalk.
4, Setting different wiring layers
And setting the plane layer reasonably, and setting different wiring layers for signals of different speeds. It is also a good way to deal with crosstalk.
5, Impedance matching
It is also possible to greatly reduce the magnitude of crosstalk. If the transmission line near-end or far-end termination impedance matches the transmission line impedance.
During the using of the circuit board, the pad peeling often occurs. Especially when the circuit board is reworked, when the soldering iron is used, the phenomenon of the pad falling off occur easily. The circuit board factory has some analysis for the falling off of the pad in this text, but also take corresponding countermeasures for the reasons.
Analysis of the reason why the pad is easy to fall off during circuit board soldering. Substrate quality problems. Since the adhesion of the resin glue between the copper foil of the copper clad laminate and the epoxy resin is relatively poor, even if the copper foil of the large-area copper foil is slightly heated or under mechanical external force, it is very easy to be separated from epoxy resin, causing problem such as peeling of the pads and peeling of the copper foil.
The influence of storage conditions of the circuit board. Affected by the weather or stored in a humid place for a long time, the moisture absorption of the circuit board is too high. In order to achieve the desired welding effect, the heat of the volatilization should be compensated for the smt welding , and the temperature and time of the welding must be extend. Such soldering conditions tend to cause the wiring board copper foil to be layered with the epoxy resin.
Soldering iron welding problem, the general circuit board adhesion can meet the ordinary welding, there will be no peeling off the phenomenon of the pad, but the electronic products are generally need to be repaired, the repair is generally repaired with electric soldering iron, due to the local high temperature of the soldering iron often reaches a temperature of 300-400 degrees, causing the local temperature of the pad to be too high, and the resin glue under the copper foil of the pad is detached at a high temperature, and the pad is detached. When the soldering iron is removed, it is easy to attach the physical force of the soldering iron to the pad, which is also the cause of the falling off of the pad.
For the pad to fall off easily under the conditions of usage, the circuit board factory adopts the following aspects to improve the soldering times resistance of the circuit board pad as much as possible to meet the needs of customers.
1: CCL is made of substrate with quality assurance. Generally, the selection of the glass fiber cloth of the copper clad laminate and the pressing technics can ensure that the soldering resistance of the printed circuit board meets the requirements of the customer.
2: The circuit board is vacuum packed before leaving the factory, and the desiccant is placed to keep the circuit board in a dry state. To reduce virtual welding and improve solderability.
3: For the thermal impact of the soldering iron on the pad, we increase the thickness of the pad copper foil by electroplating as much as possible, so that when the soldering iron is heated to the pad, the thermal conductivity of the copper foil thick pad is obviously enhanced, effectively lower local temperature of the pad , and at the same time, the fast heat conduction makes the pad easier to disassemble. The solder resistance of the pad is reached.
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