The details that should be paid attention to when PCB soldering

After the copper clad laminate is processed to produce PCB board, various through holes, and assembly holes, various components are assembled. After assembling, in order to make the components reach the connection with each circuit of the PCB, it is necessary to carry out the Xuan welding process. Brazing is divided into three methods: wave soldering, reflow soldering and manual soldering. The socket-mounted components are generally connected by wave soldering; the brazing connection of surface-mounted components generally uses reflow soldering; individual components and components are individually manual (electric chrome) due to installation process requirements and individual repair welding. Iron) welding.


1. Solder resistance of copper clad laminate

Copper clad laminate is the substrate material of PCB. During brazing, it encounters the contact of high-temperature substances in an instant. Therefore, the Xuan welding process is an important form of “thermal shock” to the copper clad laminate and a test of the heat resistance of the copper clad laminate. Copper clad laminates ensure the quality of their products during thermal shock, which is an important aspect of assessing the heat resistance of copper clad laminates. At the same time, the reliability of the copper clad laminate during Xuan welding is also related to its own pull-off strength, peel strength under high temperature, and moisture and heat resistance. For the brazing process requirements of copper clad laminates, in addition to the conventional immersion resistance items, in recent years, in order to improve the reliability of copper clad laminates in Xuan welding, some application performance measurement and assessment items have been added. Such as moisture absorption and heat resistance test (treatment for 3 h, then 260℃ dip soldering test), moisture absorption reflow soldering test (placed at 30℃, relative humidity 70% for a specified time, for reflow soldering test) and so on. Before the copper clad laminate products leave the factory, the copper clad laminate manufacturer shall perform a strict dip solder resistance (also known as thermal shock blistering) test according to the standard. Printed circuit board manufacturers should also detect this item in time after the copper clad laminate enters the factory. At the same time, after a PCB sample is produced, the performance should be tested by simulating wave soldering conditions in small batches. After confirming that this kind of substrate meets the user’s requirements in terms of resistance to immersion soldering, the PCB of this kind can be mass-produced and sent to the complete machine factory.

The method for measuring the solder resistance of copper clad laminates is basically the same as the international (GBIT 4722-92), the American IPC standard (IPC-410 1), and the Japanese JIS standard (JIS-C-6481-1996). The main requirements are:

①The method of arbitration determination is “floating soldering method” (the sample floats on the soldering surface);

②The sample size is 25 mm X 25 mm;

③If the temperature measurement point is a mercury thermometer, it means that the parallel position of the mercury head and tail in the solder is (25 ± 1) mm; the IPC standard is 25.4 mm;

④The depth of the solder bath is not less than 40 mm.

It should be noted that: the temperature measurement position has a very important influence on the correct and true reflection of the level of dip solder resistance of a board. Generally, the heating source of soldering tin is at the bottom of the tin bath. The greater the (deeper) the distance between the temperature measurement point and the surface of the solder, the greater the deviation between the temperature of the solder and the measured temperature. At this time, the lower the temperature of the liquid surface is than the measured temperature, the longer the time for the plate with dip solder resistance measured by the sample float welding method to bubble.

2. Wave soldering processing

In the wave soldering process, the soldering temperature is actually the temperature of the solder, and this temperature is related to the type of soldering. The welding temperature should generally be controlled below 250’c. Too low welding temperature affects the quality of welding. As the soldering temperature increases, the dip soldering time is relatively significantly shortened. If the soldering temperature is too high, it will cause the circuit (copper tube) or the substrate to blistering, delamination, and serious warpage of the board. Therefore, the welding temperature must be strictly controlled.

Three, reflow welding processing

Generally, the reflow soldering temperature is slightly lower than the wave soldering temperature. The setting of reflow soldering temperature is related to the following aspects:

①The type of equipment for reflow soldering;

②The setting conditions of line speed, etc.;

③The type and thickness of the substrate material;

④ PCB size, etc.

The set temperature of reflow soldering is different from the PCB surface temperature. At the same set temperature for reflow soldering, the surface temperature of the PCB is also different due to the type and thickness of the substrate material.

During the reflow soldering process, the heat resistance limit of the substrate surface temperature where the copper foil swells (bubbles) will change with the preheating temperature of the PCB and the presence or absence of moisture absorption. It can be seen from Figure 3 that when the preheating temperature of the PCB (the surface temperature of the substrate) is lower, the heat resistance limit of the substrate surface temperature where the swelling problem occurs is also lower. Under the condition that the temperature set by reflow soldering and the preheating temperature of reflow soldering are constant, the surface temperature drops due to the moisture absorption of the substrate.

Four, manual welding

In repair welding or separate manual welding of special components, the surface temperature of electric ferrochrome is required to be below 260℃ for paper-based copper clad laminates, and below 300℃ for glass fiber cloth-based copper clad laminates. And as far as possible to shorten the welding time, the general requirements; paper substrate 3s or less, glass fiber cloth substrate is 5s or less.