Influence of solder alloy composition ratio on PCB manufacturing
During pcb manufacturing,the distribution ratio of the solder alloy composition and the influence of impurities on the welding quality are great.
As we all know, RB160M-40 TE-61 changes the proportion of alloys in tin pots with more and more impurities as the working time increases during pcb manufacturing. The change in solder composition affects the soldering temperature and the viscosity, fluidity, surface tension, and wettability of the liquid solder. Long-term loss of management and maintenance of the solder in the tin pot will inevitably lead to changes in melting point, viscosity, and surface tension, resulting in unstable wave soldering quality, and tin must be changed in severe cases.
(1) Effect of solder alloy ratio on soldering temperature
A decrease in the proportion of Sn increases the melting point, and as the melting point temperature increases, the optimum soldering temperature becomes higher and higher during pcb manufacturing. As the soldering time is extended, the proportion of Sn will become less and less. Therefore, the ratio of Sn-Pb alloy is a frequently tested item.
(2) Effect of impurities such as Cu on welding quality
The solid metal immersed in the liquid solder will cause dissolution, which is called leaching. In wave soldering, copper on the PCB pads and pins is continuously dissolved into the solder, so impurities such as Cu increae over time during pcb manufacturing. When Cu is dissolved in the solder, a sheet-like intermetallic compound Cu6SI15 is formed. As the Cri6Sn5 increases, the viscosity of the solder increases, and the melting point of the solder increases. When the Cu content exceeds 1%, the fluidity is deteriorated, and the solder joint is liable to cause defects such as projection, bridging, etc., so the copper content is also a frequently tested item.
Lead-free wave soldering is about 30 °C higher than lead wave soldering, and Cu dissolves faster, so it is necessary to check the Cu content every month during pcb manufacturing.
As the wave soldering time increases, other microimpurity may be mixed in, such as zinc (Zn), aluminum (AI), cadmium (Cd), antimony (Sb), iron (Fe), antimony (Bi), and arsenic (As). ), metal elements such as phosphorus (P). Which have an impact on solder joint quality.
Solder maintenance in tin furnace
Through the above analysis, the importance of solder maintenance in the tin furnace can be recognized. The Sn-Pb ratio and impurity content should be checked regularly during pcb manufacturing, especially the copper content in the solder. Once the standard is exceeded, the excess copper-tin alloy should be removed in time. The main measures are as follows.
1 .When the proportion of Sn is reduced, some pure Sn can be appropriately added to adjust the Sn-Pb ratio.
2.Sn-Pb solder adopts the so-called “freeze-drying” method: the solder in the solder pot is cooled to 188-190 ° C for 8 h; since the density of CL16Sn5 is 8.28 g/cm 3 , the density of Sn-Pb is 8.8-8.9 g / Cm3, Cu6Sn5 is on the surface, so stainless steel wire meshmade spoon is available.
3. In lead-free wave soldering, the density of Cu6Sn5 is larger than that of lead-free solder, and Cri6Sn5 will sink at the bottom of the tin bath. Some organizations have lowered the temperature to about 235 ° C (about 8 ° C higher than the melting point temperature) during pcb manufacturing, and the tin bath was shut down overnight. At this time, most of the alloy is still in a molten state, and special tools can be designed to remove precipitated Cu6Sn5 from the bottom of the tin bath, but the difficulty is still very great.
4.Strengthen the daily maintenance of the equipment.
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