How do I get around problem of dropped pieces in the secondary return£¿

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Double-sided mounting process can improve the actual utilization of PCB, reduce manufacturing costs, to meet the demand for miniaturization and high integration of end products. However, the use of double-sided mounting, double-sided reflow process will bring the following problems.
One is the second reflow, the bottom of the larger or heavier components may fall because of gravity or vibration, and the second is the bottom of the solder joints will affect the reliability of the solder joints because of the melting again.
In order to solve the problem of component dropping, there are generally the following methods:
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1. Choose a solder paste with higher viscosity. For most components, the second reflow, the surface tension of the solder joints molten solder paste can maintain the components on the bottom of the viscous force, so that the components are firmly fixed on the substrate? Obviously, there is a component quality and pin (pad) tension of the proportionality of the relationship, some research conclusions show that when the component on the bottom of the viscous force ¡Ý 4.65g / cm2 will not fall.
2. Using high-temperature + low-temperature solder paste over the reflow oven. That is, in the first reflow printing high-temperature solder paste, the second reflow when the use of low-temperature solder paste, to avoid the first reflow soldering point solder paste re-melting. This method should pay attention to the warping and pad offset problems brought about by high-temperature soldering.
3. For the heavier quality components selected in the second soldering over reflow.
Shenzhen Fitech Co., Ltd. developed FL170/180/200 series of micro-nanoparticle-enhanced low-temperature high-strength solder, the use of micro-nanoparticle-enhanced particles dispersed in the solder, refining the organisational structure, inhibit the enrichment of Bi, the reliability of the product has been significantly improved. The product has been patented in the United States and Japan.
In the first reflow choose SAC305, peak temperature 245 ¡æ, melting point 217 ~ 219 ¡æ; the second reflow peak temperature optional FL170/180/200, can achieve 170 ~ 200 ¡æ low-temperature soldering, suitable for non-heat-resistant components and thin substrate chips.
Fitech secondary reflow solution has the following advantages:
1. Effectively solve the low temperature welding reliability pain point. Push and pull up 30%, close to SAC305.
2. Effectively reduce the warpage and pad shift problems brought about by high temperature soldering, reduce the open circuit, false soldering and other failure problems, improve yields.
3. Save energy, energy saving and emission reduction, improve efficiency.

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