PCB News - Analysis and Improvement of Causes of Components Easily Falling Off after Immersion Gold Circuit Board Welding

Analysis and Improvement of Causes of Components Easily Falling Off after Immersion Gold Circuit Board Welding

Problem Description:

After the customer solders our company's gold circuit board, the soldered device is very easy to fall off.Shenzhen circuit board manufacturers attach great importance to it, check the production records of the batch at that time, find out the empty circuit boards of the batch, and send them to the tin spraying factory to spray tin to test the tinning effect of the board, and at the same time, send the empty boards The patch factory did lead and lead-free patch experiments to test the actual patch effect, and no problems were found. 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 mount technology 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. Remedial measures for circuit boards returned by customers.

3. Slice analysis of the soldering part of the circuit board returned by the customer.

4. Return the stock board to the spray tin factory to spray tin to test the solderability of the immersion gold on the circuit board.

5. The stock board is brushed with solder paste and reflow soldered to test the actual solderability of the circuit board.

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 comes into contact with 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, such as 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 mass board patch production, the first board production inspection is not carried out, and it is very troublesome to find the problem after all of it is completed.

B. The active ingredients contained in the solder paste produce an effect and volatilize when the high temperature of the reflow soldering is performed for the first time, and an effective alloy layer is not formed. The effect of high-temperature soldering again will not be obvious, which will cause unfavorable conditions for the remedy.

Temporary remedy:

Because it is a batch of immersion gold circuit boards, manual repair welding with electric soldering iron and manual repair welding with hot air guns are not practical. So although useful, it cannot 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 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.