PCB Technical - Analysis and improvement of the reason why the device is easy to fall off after the soldering of the gold plate

Analysis and improvement of the reason why the device is easy to fall off after the soldering of the gold plate

Problem Description:
After the customer welds our immersion gold plate, the welded device is very easy to fall off. We attach great importance to checking the production records of the batch at that time, finding out the empty boards of the stock circuit boards of this batch, and sending them to the tin spraying factory to spray tin to test the tinning effect of the boards, and at the same time, sending the empty boards to the placement factory Do lead and lead-free patch experiments, test the actual patch effect, and found no problems. The customer sent the problem board back in kind, and we checked the real thing, and it was true that the device was very easy to fall off. Check the falling point, there is a matt black substance.
Because this undesirable phenomenon involves the surface treatment of the circuit board, solder paste, reflow soldering and SMT surface mounting process and other processes, in order to accurately find the cause, we convened the sinking gold factory, the patch factory, the solder paste factory and The craft department of our company will study together to find out what the problem is.

Analysis of the cause of the problem:
Our company found this circuit board produced in the same batch in stock. Do analysis experiments to find out the problem:
1: Immersion gold slice analysis of the problem board:
2: Return the stock board to the spray tin factory to spray tin to test the solderability of the immersion gold on the circuit board.
3: The stock board is brushed with solder paste, reflow soldered, and the actual solderability of the circuit board is tested.
4: Analysis of the welding part of the circuit board returned by the customer.
5: Remedial measures for the circuit board returned by the customer.

Through a series of experimental analysis and judgment, the surface of the copper foil of the circuit board is plated with a layer of nickel, and then a layer of gold is deposited on the nickel layer to protect the nickel layer from oxidation. During patch soldering, first put a layer of solder paste on the pad. The solder paste naturally penetrates the gold-immersion layer and contacts the nickel layer (the dull black is the result of the effect of the solder paste and the nickel layer). The solder paste contains some active ingredients, When the temperature reaches the melting temperature of the solder paste during reflow soldering, with the help of the active ingredient, the tin forms a metal compound layer (IMC) with each layer.
The board returned by the customer did not meet the soldering requirements at the time of soldering. For example, it did not reach the reflow soldering temperature (the northern temperature is low, the preheating is not enough, the actual temperature is inconsistent with the temperature zone table), the activity of the solder paste (tin Paste storage conditions), the thickness of the steel mesh, etc., causing the nickel layer to fail to form a metal compound layer with tin, resulting in the device falling off.
There are two regrets:
A: During the batch production, the first board production inspection is not carried out, and it is very troublesome to discover the problem after all of it is completed.
B: The active ingredients contained in the solder paste produce effects and volatilize during the first high temperature reflow soldering, and an effective alloy layer is not formed. The effect of high-temperature soldering again will not be obvious, which will cause adverse conditions for the remedy.

Temporary remedy:
Because it is a batch board, repair welding with a manual electric iron and manual repair with a hot air gun are not practical. So although useful, it cannot solve the problem.
Increase the furnace temperature and reflow soldering. Although the active components of the solder resist in the solder paste are lost, the metal compound layer can also be formed at a sufficiently high temperature. As for the effect and high temperature loss, please ask the customer to balance it.
We tested that under the condition of not brushing the flux,
The problem 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 the reflow soldering to 285 degrees, and perform the reflow soldering again. The result shows that the device still falls off.
Increase the temperature of reflow soldering to 300 degrees again, and reflow soldering again, the result is firm soldering.
Will the effect of repairing flux on the problem board be better? If the customer needs it, we can arrange the placement factory to do the test again.

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