The printed circuit board is often referred to as PCB. PCB component is called PCB assembly service. Components on board can be SMT (surface mount) components, plug-in components, pressure components, or assembly parts. Today, take a common motherboard as an example to share the assembly process and common problems of the PCBA factory.
After the ICT test station, it needs to go through an assembly station to install the button battery, radiator, CPU base bracket, CPU backplane, handle, some isolating Mylar, the plastic column for limit, screw, etc. Figure 1 shows the server motherboard with assembly.
When assembling parts, some external forces should be applied to PCBA, so PCBA should be fixed and supported on the assembly fixture before assembly. Some PCBA need to assemble the bottom surface of the PCBA board, so two supporting fixtures need to be made. The support fixture also needs to pass electrostatic protection and stress test before delivery to the production line. The standard of electrostatic protection can refer to ANSI / ESD s20.20. Refer to IPC-JEDEC-9704a (L) for the stress test.
The following describes the process and common problems of PCB assembly service according to the type of components.
PCB, also known as a printed circuit board, is the electrical connection of electronic components. We should all have seen PCB, but we don’t know much about it. Do you want to know how to solder PCB assembly service? What should we pay attention to when soldering PCB? How to install a PCB radiator?
Ideaspcb is a professional PCB assembly service factory with more than 20 years of production experience.
[PCB welding methods and precautions]
At present, factories that provide PCB assembly service usually use visual soldering machine to weld. To better use visual soldering machines, it is necessary to know the precautions of PCB board and soldering machine to achieve a better soldering effect. So how does the visual soldering machine work when soldering PCB? The following is the explanation of IDEASPCB, a professional PCBA factory:
How to weld PCB with a visual soldering machine?
First of all, the surface of the weldment should be clean. To achieve a good combination of the solder and the weldment, the weldment’s surface must be kept clean. Even for weldments with good weldability, if there is an oxide layer, dust, and oil on weldments’ surface. Be sure to be clean before soldering. Otherwise, the formation of the alloy layer around the weldment will be affected, so It can not guarantee soldering quality.
Secondly, weldments should be weldable. The quality of tin welding mainly depends on the ability of solder to wet the surface of the weldment; that is, the wettability of two metal materials is weldability. If the weldability of the weldment is poor, it is impossible to weld qualified solder joints. Solderability refers to the good bonding performance of solder and solder under appropriate temperature and flux.
Then, the setting of soldering time should be appropriate. Soldering time refers to the time required for physical and chemical changes in the soldering process. It includes the time for the solder to reach the solder temperature, the time for the solder to melt, the time for the flux to function, and the time for the metal alloy to form.
The soldering time of the circuit board should be appropriate. If it is too long, it is easy to damage the soldering parts and devices. If it is too short, it can not meet the requirements.
Which common PCB assembly service electronic components need to be welded?
With the continuous improvement of technology, some of the above-mentioned PCB electronic components are wave soldering or chip placement, with high efficiency and more reliable stability. The rest of the components can’t pass through the furnace, and can’t be pasted. It can only be solved after the automatic soldering machine. There are many practical cases of PCB welding. Usually, the Engineering Department of a factory will improve the process and require more stable and reliable welding methods. Their starting point is to require more stable quality. efficient.
Take the medical B-ultrasound motherboard as an example. There are hundreds of electronic components on the PCB assembly service board, and the total number of solder joints is hundreds. Manual operation is really cumbersome. Solder leakage often occurs, and the tin content is not well controlled, which leads to bad solder connection. To solve these problems, they found our automatic soldering machine. According to our understanding of PCB pads and electronic components. What kind of tin wire to use, the specific pad position, the amount of tin to be produced by the automatic soldering machine, and the heating temperature will be strictly set according to the parameters.
You can master the detailed position change of each solder joint in PCB assembly service, including the way of solder head down of automatic soldering machine, including whether the solder pad is required to be penetrated or not. You will strictly set and compare the parameters and use the actual effect to verify this solder joint’s quality and efficiency. The position of the automatic soldering machine also plays an important role in PCB welding. Okada technology has researched and developed automatic soldering machines for many years. For PCB automatic soldering, Okada technology has the rich practical experience and can fulfill the process requirements of each solder joint one by one according to the requirements of customers.
In PCB assembly service, we often find that the automatic soldering machine can’t weld well. Ideaspcb makes some conventional summary: in common cases, the PCB pad is covered by green oil, which can be welded well with an ordinary constant temperature soldering iron. However, due to the short time of the automatic soldering machines contacting the pad, many cases lead to bad pad welding. This problem can only be solved by requiring PCB manufacturers to improve.
About the problem of PCB pad oxidation:
Oxidation of gold-plated plates was not common before, but now it is widespread. The main reason is that the price has gone up now. Still, PCB processing price has not gone up much, so the PCB production industry has to plate thinner and thinner. The gold cylinder’s maintenance is not good; the impurity content is large, so the probability of oxidation increases when the weather is bad. Oxidation can be divided into the following two cases:
First, the nickel surface has not been oxidized before gold plating. If this happens, there is basically no way to deal with it. The only way is to remove the gold layer and nickel layer. I know that there is a liquid medicine that can do it without damaging the copper surface, but the price is higher;
The second is the oxidation after gold plating. The oxidation is that the nickel and copper ions in the gold cylinder exceed the standard, or when the gold plating time is only 3-5 seconds, the gold layer does not cover the nickel surface, causing the bottom nickel oxidation.
So oxidation for automatic soldering machine welding situation and instability, some are difficult to do with flux. In general, after wave soldering or chip placement, the next post soldering process will be carried out immediately. After decades of soldering experience, this problem has been successfully solved.
About PCB assembly service’s electronic components?
If the cutting foot of electronic components on the PCB board is too long, it will likely block the position under the soldering iron head and resist the soldering head of the automatic soldering machine. As a result, the soldering is not good, and the cutting is too short, and it is also easy to cause defects. For example, some manufacturers of automatic soldering machines do not need tin penetration or need efficiency, and sometimes there are false soldering sites. These practical problems are in the case of automatic soldering machines. In fact, everyone has seen it.
Installation method of PCB radiator
What is the installation method of PCB assembly service radiator? Many communication products have high power consumption, so they need to use a heat sink for heat dissipation. There are two common fixing methods of radiator: mechanical fixing and glue fixing.
Common design defects mainly include adhesive radiator falling off, screw installation radiator leading to PCB bending, and even device failure, especially BGA Solder joint failure.
(1) what shall fix the radiator by mechanical means? what shall adopt elastic installation method? It is strictly forbidden to fix with inelastic screws.
(2) It is not recommended that multiple chips share a common heat sink, especially BGA chips. It will collapse freely during welding, and its height from the PCB surface is difficult to control. Therefore, when multiple chips share a common heat sink, the heat sink can only be installed and fixed with a thermally conductive rubber pad and heat sink mechanically. However, this design, due to the installation sequence of screws during the installation operation, often leads to flattening or fracture of BGA Solder Joints.
(3) When using the adhesive process, It should consider matching adhesive area and radiator quality and the wettability between adhesive and radiator surface (for example, some adhesive is not wettable with Ni coating). Otherwise, it is easy to fall off.