PCB Tech - Double-sided circuit board welding method

01--Circuit board welding skills

Soldering circuit board skill 1:

The selective soldering process includes: flux spraying, circuit board preheating, dip soldering and drag soldering. Flux coating process In selective soldering, the flux coating process plays an important role.

When the soldering heating and soldering are finished, the flux should have sufficient activity to prevent the generation of bridges and prevent the circuit board from oxidizing. Flux spraying is carried by the X/Y manipulator to carry the circuit board through the flux nozzle, and the flux is sprayed onto the soldering position of the PCB circuit board.

Soldering circuit board skill 2:

For the microwave peak selective soldering after the reflow soldering process, it is important that the flux is sprayed accurately, and the micro-hole spray type will not contaminate the area outside the solder joints.

The diameter of the micro-spot sprayed flux dot pattern is greater than 2mm, so the position accuracy of the sprayed flux deposited on the circuit board is ±0.5mm to ensure that the flux is always covered on the welded part.

Circuit board soldering technique 3:

The process characteristics of selective soldering can be understood by comparing with wave soldering. The obvious difference between the two is that the lower part of the circuit board is completely immersed in liquid solder in wave soldering, while in selective soldering, there are some specific areas and solder waves. touch.

Since the circuit board itself is a poor heat transfer medium, it will not heat and melt the solder joints adjacent to the components and the circuit board area during soldering.

The flux must also be pre-coated before soldering. Compared with wave soldering, the flux is coated on the lower part of the circuit board to be soldered instead of the entire PCB circuit board.

In addition, selective soldering is suitable for the soldering of plug-in components. Selective soldering is a new method. A thorough understanding of selective soldering processes and equipment is necessary for successful soldering.

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02--Precautions for circuit board welding

Remind everyone that after getting the bare PCB board, you should first perform an appearance inspection to see if there are short-circuits, open circuits, etc., and then familiarize yourself with the schematic diagram of the development board, and compare the schematic with the PCB silk screen layer to avoid discrepancies between the schematic and the PCB.

After the materials required for PCB welding are prepared, the components should be classified. All components can be divided into several categories according to their size to facilitate subsequent welding. Need to print a complete list of materials. In the welding process, if one item is not completed, use a pen to cross out the corresponding option, which is convenient for subsequent welding operations.

Before welding, take anti-static measures such as wearing a static ring to avoid damage to the components caused by static electricity. After the equipment required for welding is ready, the tip of the soldering iron should be kept clean and tidy. It is recommended to use a flat-angle soldering iron for the first soldering. When soldering components such as 0603 packaged components, the soldering iron can better contact the pads and facilitate soldering. Of course, for masters, this is not a problem.

When selecting components for soldering, the components should be soldered in the order from low to high and from small to large. In order to avoid the welding of smaller components caused by welding of larger components. Priority is given to soldering integrated circuit chips.

Before welding the integrated circuit chip, it is necessary to ensure that the chip placement direction is correct. For the chip silk screen layer, generally rectangular pads indicate the starting pins. When soldering, fix one pin of the chip first, fine-tune the position of the component, and fix the diagonal pin of the chip, so that the component is accurately connected and then soldered.

SMD ceramic capacitors and Zener diodes in voltage regulator circuits have no positive and negative poles. Light-emitting diodes, tantalum capacitors and electrolytic capacitors need to distinguish between positive and negative poles. For capacitors and diode components, generally the end with an obvious mark should be negative. In the package of SMD LED, the direction along the lamp is the positive-negative direction. For packaged components marked as diode circuit diagram by silk screen, the negative end of the diode should be placed at the end with a vertical line.

For crystal oscillators, passive crystal oscillators generally have only two pins, and there is no difference between positive and negative. Active crystal oscillators generally have four pins. Pay attention to the definition of each pin to avoid soldering errors.

For the welding of plug-in components, such as power supply module related components, the device pins can be modified before welding. After the components are placed and fixed, the solder is generally melted by a soldering iron on the back and then merged into the front by the pad. There is no need to put too much solder, but the components should be stable first.

The PCB design problems found during the soldering process should be recorded in time, such as installation interference, incorrect pad size design, component packaging errors, etc., for subsequent improvements.

After soldering, use a magnifying glass to check the solder joints to check whether there is any false soldering or short circuit.

After the circuit board welding is completed, the surface of the circuit board should be cleaned with a cleaning agent such as alcohol to prevent the iron filings attached to the surface of the circuit board from short-circuiting the circuit, and it can also make the circuit board cleaner and more beautiful.

03--Characteristics of double-sided circuit board

The difference between single-sided circuit boards and double-sided circuit boards is the number of copper layers. Popular science: double-sided circuit boards have copper on both sides of the circuit board, which can be connected through vias. However, there is only one layer of copper on one side, which can only be used for simple circuits, and the holes made can only be used for plug-in connections.

The technical requirements for double-sided circuit boards are that the wiring density becomes larger, the aperture is smaller, and the aperture of the metallized hole becomes smaller and smaller. The quality of the metallized holes on which the layer-to-layer interconnection relies is directly related to the reliability of the printed board.

With the shrinking of the pore size, the debris that did not affect the larger pore size, such as brush debris and volcanic ash, once left in the small hole will cause the electroless copper and electroplating to lose its effect, and there will be holes without copper and become holes. Metallization is deadly.

04--Welding method of double-sided circuit board

Double-sided circuit board In order to ensure the reliable conductive effect of double-sided circuit, it is recommended to weld the connection holes on the double-sided board with wires or the like (that is, the through-hole part of the metallization process), and cut off the protruding part of the connection line to avoid Injury the operator's hand, this is the preparation for the connection of the board.

The essentials of double-sided circuit board welding:

For devices that require shaping, they should be processed in accordance with the requirements of the process drawings; that is, they must be shaped first and then plug-in

After shaping, the model side of the diode should face up, and there should be no discrepancies in the length of the two pins.

When inserting devices with polarity requirements, pay attention to their polarity not to be reversed. After inserting, roll integrated block components, no matter it is a vertical or horizontal device, there must be no obvious tilt.

The power of the soldering iron used for soldering is between 25~40W. The temperature of the soldering iron tip should be controlled at about 242 degree Celsius. If the temperature is too high, the tip is easy to "die", and the solder cannot be melted when the temperature is low. The soldering time should be controlled within 3~4 Second.

The formal welding is generally performed according to the welding principle of the device from short to high and from the inside out. The welding time must be mastered. If the time is too long, the device will be burnt, and the copper line on the copper clad board will also be burnt.

Because it is double-sided soldering, a process frame or the like for placing the circuit board should also be made, so as not to squeeze the components underneath.

After the circuit board soldering is completed, a number of check-in-place inspections should be carried out in many ways to find out where there are missing plugs and solders. After confirmation, trim the redundant device pins and the like on the circuit board, and then flow into the next process.

In the specific operation, the relevant process standards should also be strictly followed to ensure the welding quality of the product.

With the rapid development of high technology, electronic products that are closely related to the public are constantly being updated. The public also needs electronic products with high performance, small size and multiple functions, which puts forward new requirements on circuit boards. This is why the double-sided circuit board was born. Due to the wide application of double-sided circuit boards, the manufacture of printed circuit boards has also become lighter, thinner, shorter and smaller.