Whether reflow welding, wave welding or hand welding, surface tension is a disadvantage to forming a good solder joint. However, surface tension can be utilized in SMT patch processing and reflow welding -- Self Alignment effect can be produced under the action of balanced surface tension when the solder paste reaches the melting temperature. Next, Shenzhen PCBA processing manufacturer - Shenzhen kingford will introduce the effect and improvement measures of surface tension and viscosity in PCBA processing.
Effect and improvement of surface tension in PCBA processing
First, the role of surface tension in welding
Surface tension is in the opposite direction of wetting force, so surface tension is one of the factors unfavorable to wetting.
Whether reflow welding, wave welding or hand welding, surface tension is a disadvantage to forming a good solder joint. But the surface tension can be used in SMT patch machining and reflow welding.
When the solder paste reaches the melting temperature, Self Alignment effect occurs under the action of balanced surface tension. In other words, when the component placement position deviates slightly, the component can be automatically pulled back to the approximate target position under the action of surface tension.
Therefore, the surface tension makes the reflow process more relaxed in the mounting accuracy requirements, it is easier to achieve a high degree of automation and high speed.
Meanwhile, due to the characteristics of "re-flow" and "self-positioning effect", SMT re-flow welding process has stricter requirements for pad design, component standardization and other aspects.
If the surface tension is not balanced, even if the mounting position is very accurate, there will be component position deviation, monument, bridge and other welding defects after welding.
Wave soldering, due to the size and height of the SMC/SMD component body itself, or because the high component block the high component and block the oncoming tin wave flow, and by the influence of the tin wave flow surface tension caused by shadow effect, the formation of liquid solder on the back of the component body can not be infiltrated into the flow block area, resulting in leakage welding.
2. Measures to change the surface tension
Viscosity and surface tension are important properties of solder. Good solder should have low viscosity and surface tension when melting. Surface tension is the nature of matter, can not be eliminated, but can be changed.
The main measures to reduce the surface tension and viscosity in PCBA welding are as follows:
Step 1 Raise the temperature. Increasing the temperature can increase the molecular distance in the molten solder and reduce the attraction of the molecules in the liquid solder to the surface molecules. So heating can reduce viscosity and surface tension.
2. Adjust the proportion of metal alloys. The surface tension of Sn is very high, and the addition of Pb can reduce the surface tension. As can be seen from the figure, when the content of lead is increased in Sn-Pb solder, the surface tension decreases obviously when the content of Pb reaches 37%.
When the content of lead is increased in Sn-Pb solder, the surface tension decreases obviously when the content of Pb reaches 37%.
3. Add active agent. This can effectively reduce the solder surface tension, also can remove the solder surface oxide layer.
4. Improve the welding environment. Nitrogen protected PCBA welding or vacuum welding can reduce high temperature oxidation and improve wettability.
3. The result of the destruction of PCBA processing surface tension
In PCBA processing, when the solder surface tension is damaged, it will cause poor welding of surface mount components. The main phenomenon of poor wetting is that, in the welding process, there is no reaction between the substrate welding area and the solder after wetting, resulting in less welding or welding leakage.
The main causes of poor wetting are:
1. The surface of the welding area is contaminated, the surface of the welding area is stained with flux, or metal compounds are generated on the surface of the patch element. Can cause poor wetting. Such as sulfide on the surface of silver, tin has oxides on the surface will produce poor wetting.
2, when the residual metal in the solder exceeds 0.005%, the activity of the flux is reduced, and poor wetting will also occur.
3, wave soldering, there is gas on the surface of the substrate, but also easy to produce poor wetting.
The solutions to poor wetting are:
1. Strictly implement the corresponding welding process.
2, PCB board and component surface to do a good job of cleaning.
3, select the appropriate solder, and set a reasonable welding temperature and time.
Choose Shenzhen kingford to do PCBA processing four reasons
1. Strength guarantee
▪SMT workshop: We have imported SMT machines and several sets of optical inspection equipment, with a daily output of 4 million. Each process is equipped with QC personnel, who can keep an eye on product quality.
▪DIP production line: We have two wave-soldering machines, among which there are more than 10 old employees who have worked for more than three years. The skilled workers can weld all kinds of plug-in materials.
2. Quality assurance, cost-effective
▪ High-end equipment can stick precision shaped parts, BGA, QFN, 0201 materials. Can also template patch, loose material hand.
▪ Sample and size batch can be produced, proofing from 800 yuan, batch 0.008 yuan/point, no start-up fee.
3. Rich experience in SMT and welding of electronic products, stable delivery
▪ Accumulated SMT SMT processing services for thousands of electronic enterprises, involving many kinds of automotive equipment and industrial control motherboard. The products are often exported to Europe and the United States, and the quality can be affirmed by new and old customers.
▪ On time delivery, normal 3-5 days after complete materials, small batch can also be expedited on the same day shipment.
4. Strong maintenance ability and perfect after-sales service
▪ Experienced maintenance engineers can repair all kinds of patch welding caused by bad products, to ensure the connection rate of each piece of circuit board.
▪ 24-hour customer service staff at any time response, the fastest speed to solve your order problems.