1. Lead-free process requirements for flux
(A) Since there is a chemical reaction between the flux and the surface of the alloy, different flux components are selected for different alloy compositions.
(B) Since the wettability of the lead-free alloy is poor, the flux activity is required to be high.
(C) To increase the activation temperature of the flux, it is necessary to adapt to the lead-free high temperature soldering temperature.
(D) Less residue after soldering and non-corrosive, meeting ICT probe capability and electromigration.
2. The key to solder paste printability and solderability is flux.
After determining the lead-free alloy, the key is the flux. Select the solder paste to do the process test to see if the printability meets the requirements and how the quality is after soldering. In short, choose a solder paste that suits your product and process.
3, lead-free flux must be specially formulated in the solder paste flux is a key material to purify the solder surface, improve wettability, prevent solder oxidation and ensure solder paste quality and excellent processability. At high temperatures, the flux acts as a cleaning agent on the pads of the PCB, on the ends of the components and on the surface of the leads, and at the same time activates the metal surface.
4. No VOC-free cleaning in wave soldering. The flux is also specially prepared. Lead-free solder pastes and wave soldered water soluble fluxes are also required for certain products.
5, the adverse effects of high temperature on components
Ceramic resistors and special capacitors are very sensitive to the slope of the temperature curve (the rate of change of temperature). Because the thermal expansion coefficient CTE of the ceramic body and the PCB are large, the component body and solder joint cracks are easily caused when the solder joint is cooled, and the component cracking phenomenon The difference in CTE, temperature, and size of components are directly proportional.
Aluminum electrolytic capacitors are extremely sensitive to sharpness.
Connectors and other plastic package components fail significantly at high temperatures. Mainly layering, popcorn, deformation, etc., rough statistics, the temperature of the moisture sensitive element (MSL) is reduced by 1 level for every 10 °C increase in temperature. The solution is to minimize the peak temperature; dehumidification of the moisture sensitive components.
6, the adverse effects of high temperature on the PCB
The adverse effects of high temperature on the PCB have been analyzed in the third section. The high temperature is easy to thermally deform the PCB, the strength and insulation resistance value are reduced due to aging of the resin, and the metallization is caused by the mismatch of the Z-axis of the PCB and the C-axis in the XY direction. Reliability problems such as failure of the hole plating and failure.
The solution is to minimize the peak temperature. Generally, simple consumer products can use FR-4 substrates. Thick plates and complex products require high temperature resistant FR-5 or CEMn instead of FR-4 substrate.
7, electrical reliability
Flux residue formed by reflow soldering, wave soldering, and rework, in a humid environment and a certain voltage, an electrochemical reaction may occur between the conductors, resulting in a decrease in surface insulation resistance. If electromigration and dendrite (tin whisker) growth occurs, a short circuit between the wires will occur, causing a risk of electromigration (commonly known as "leakage"). In order to ensure electrical reliability, it is necessary to evaluate the performance of different no-clean fluxes.
8. About lead-free rework
1 Rework of lead-free solder is quite difficult, mainly due to:
(A) Lead-free solder alloys have poor wettability.
(B) High temperature (simple PCB 235 ° C, complex PCB 260 ° C).
(C) The process window is small.
2 Lead-free rework considerations:
(A) Select the appropriate rework equipment and tools.
(B) Correctly rework equipment and tools.
(C) Correct selection of solder paste, flux, solder wire and other materials.
(D) Set the welding parameters correctly.
In addition to adapting to the high melting point and low wetting of lead-free solders. At the same time, care must be taken during the repair process to minimize any potential factors that adversely affect the reliability of components and PCBs.
9. Summary of lead-free and lead-mixed use in excessive periods.
(A) Lead-free solder and lead-free solder joints - the best results.
(B) Lead-free solder and lead soldered end - currently used, can be applied, but must control the content of Pb, Cu, etc., to prepare the corresponding flux, but also strictly control the temperature parameters and other process parameters, otherwise it will cause Reliability issues.
(C) Leaded solder and lead-free solder joints - the worst effect, BGA, CSP lead-free solder balls can not be used in the lead process, not recommended.
At present, the solder used in the lead-free process has a much larger tin content than the original solder composition, and the alloy composition is relatively improved. In the production process, the amount of tin slag produced is also greatly increased compared to the original ordinary solder. Cost control in terms of material consumption is beneficial if the amount of dross produced can be reduced.
Tin slag is mainly an oxide produced by the reaction of tin with oxygen in a high temperature environment. Most of the tin oxide can be separated by physical high temperature stirring (ie, tin slag reduction), and the separated tin can be reused or chemically replaced. The reaction is repeated by replacing the oxygen molecules in the tin slag and reducing them to pure tin.
Each factory can comprehensively consider a better lead-free road according to its own machine and process arrangement.
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