Since this bad phenomenon involves various processes such as surface treatment of circuit boards, solder paste, reflow soldering, and SMT surface mount technology, in order to find out the cause accurately, we called senyan Factory, Chip Factory, Tin Yu Factory and our company. Let's study together with the technology department of the company to find out where the problem is.
Analysis of the cause of the problem:
Our company found this circuit board produced in the same batch in stock. Do profiling experiments to find out what the problem is:
1. Analysis of the immersion gold slice of the problem board:
2. Remedial measures for circuit boards returned by customers.
3. Slice analysis of the welding part of the circuit board returned by the customer.
4. The stock board is sprayed back to the tin spraying factory to test the solderability of immersion gold on the circuit board.
5. Inventory board brush tinning, reflow soldering, test the actual solderability of the circuit board.
Through a series of experimental analysis and judgment, a layer of nickel is plated on the surface of the copper foil of the circuit board, and then a layer of gold is deposited on the nickel layer to protect the nickel layer from oxidation. When patch soldering, put a layer of tin on the pad first. The tin naturally penetrates through the immersion gold layer and contacts the nickel layer (the matte black is the result of the interaction between the tin and the nickel layer), and the tin contains some active components that have been reflowed. When soldering, when the temperature reaches the melting temperature of tin, with the help of active ingredients, tin forms a metallization layer (IMC) with each layer.
The board returned by the customer did not meet the soldering requirements when soldering at that time, such as not reaching the temperature of reflow soldering (the temperature in the north is low, the preheating is not enough, the actual temperature is inconsistent with the temperature zone table), the activity of tin (solder paste storage conditions), the thickness of the steel mesh, etc., resulting in the failure of the nickel layer to form a metallization layer with the tin, resulting in the device falling off.
There are two regrets:
A. During batch board patch production, the first board production inspection is not carried out, and it is very troublesome to find problems after all the boards are finished.
B. The active ingredients contained in the solder paste produce effects and volatilize during the first reflow soldering high temperature. An effective alloy layer is not formed, and the effect is not obvious when soldering at high temperature again, which creates unfavorable conditions for remediation.
Temporary remedies:
Because it is an immersion gold circuit board batch board, manual electric soldering iron repair welding and manual repair welding with hot air gun are not practical. So although it works, it doesn't solve the problem.
Increase the furnace temperature and reflow soldering. Although the help of the solder resist active components in the solder paste is lost, a metallization layer can also be formed at a high enough temperature. As for the effect and loss of high temperature, customers need to ask customers to balance it by themselves.
We tested under the condition of not brushing flux,
The problematic board returned by the customer was reflowed at 265 degrees, and the result showed that the device would still fall off.
Adjust the temperature of reflow soldering to 285 degrees, and reflow soldering again. The results showed that the device would still fall off. Increase the temperature of reflow soldering to 300 degrees again, and reflow soldering again, the result is that the soldering is firm.
Will the effect of brushing the flux on the problem board be better? If the customer needs it, we can arrange the placement factory to do the test again.
