What is the components tin-connection?
It is a bad phenomenon in wave soldering project in PCBA.Please see the following picture：
The main chips in the red circle in the picture are connected together, which is abnormal. Tin-connected parts are two or more solder joints connected by solder, which will lead to poor appearance and function, and is defined as a defect level by IPC-A-610D.
Reasons for the formation of tin connections
As far as we know, the components tin-connection is a kind of short circuit. It may be that the preheating temperature is not enough to cause the component to fail to meet the temperature requirements. During the soldering process, the component absorbs a large amount of heat, which leads to poor tin drag and the formation of tin. ; It is also possible that the temperature of the tin furnace is low, or the welding speed is too fast. The specific reasons are divided into the following types:
1. Flux is almost the main cause of components tin-connection.
A.The flux activity is not enough.
B. The wettability of the flux is not enough.
C. The amount of flux applied is too small.
D. Uneven coating of flux.
E. The circuit board cannot be coated with flux.
C. The amount of flux applied is too small.
D. Uneven coating of flux.
E. The circuit board cannot be coated with flux.
We all know that as long as the activity is strong enough, the oxide layer on the surface of the copper needle can be removed in a short time, but we need to ensure that the steam during the welding process will not cause subsequent corrosion of the needle (green). Strengthen the rosin or resin content in the flux.Guaranteed good carrier performance under high temperature welding.
2. The circuit board is not tinned regionally.
This problem is also the key to components tin-connection.
Therefore, when the soldering point of the circuit board is not sticky, we need to check whether there is copper skin around the soldering point, if there is any, scrape the solder resist on it and re-solder. If there is no copper skin around, it is necessary to heat the soldering point with a soldering iron, and then put the enameled wire head coated with solder in advance up to be soldered. Generally, the soldering should be heated before connecting, first dip the tin on the wire, which is much better.
3. Some pads or solder feet are seriously oxidized.
If the pads of the PCB board or the pins of the components have been seriously oxidized, it is easy to cause false soldering or components tin-connection. In fact, the component pins are oxidized, you can first use a small tin furnace to dip this batch of components first, and then solder. Secondly, choose ferrochrome with a slightly higher temperature. If the pad is oxidized, it must be cleaned or sanded once with sandpaper to remove the oxidation. After welding, the inspection position is added to ensure that the welding position is not fake.
4. The wiring of the circuit board is unreasonable (the distribution of meta-parts is unreasonable).
5. The walking direction is wrong.
6. Insufficient tin content, or copper exceeding the standard; [Excess impurities cause the melting point (liquid line) of the tin liquid to rise]
7. The foaming tube is blocked and the foaming is not uniform, resulting in uneven coating of the flux on the circuit board.
8. The setting of the air knife is unreasonable (the flux is not evenly blown).
9. The walking speed and preheating coordination are not good.
Therefore, I suggest that before re-welding, we need to use KIC to test whether the component temperature and the board surface temperature meet the welding requirements.
What improvement measures should we take when a tin connection occurs?
Please see the following 5 effective methods:
1. Design according to PCB design specifications.
The long axes of the two tip Chips are perpendicular to the welding direction, and the long axes of SOT and SOP should be parallel to the welding direction. Widen the pad of the second pin of the SOP (design a tin steal pad).
2. The shape of the pin inserted into the assembly
depends on the hole distance of the printed board and the assembly requirements. For example, if a short insertion sub soldering process is used, the component pins of the soldering surface are exposed 0.8 to 3 mm from the surface of the printed circuit board, and the component body is required during the correct insertion process.
3. Set the preheating temperature according to the PCB size
the number of layers, the number of components and the installed components.
4. The temperature of tin wave is 250 ± 5 ℃, and the welding time is 3 ~ 5s.
When the temperature is slightly lower, the conveyor speed should be slowed down
5. Replace the flux.The main component of flux is rosin.
Tin bath temperature should be kept in check as rosin decomposes at approximately 260 degrees Celsius due to tin’s reaction. Flux is a chemical substance that promotes welding. In soldering, it is an indispensable auxiliary material, and its role is extremely important. If components tin-connection appear in wave soldering, it means that we urgently need to find a new flux to change this bad phenomenon.
In addition, we suggest that in the process of wave soldering, the factory must operate according to the matters needing attention, so as to better avoid the occurrence of tin connection. The welding temperature should be controlled within the range of 250 ° c ±5 ° c,
The temperature will directly affect the welding quality
Adjust the angle of inclination of the welding fixture to the wave port to approximately 6°.Control the welding line speed between 1 and 1.6n/min.Maintain the peak height of the solder bath tin surface at around 10mm.Generally, set the peak height at 1/2 to 2/3 the thickness of the circuit board.
Too much assembly can lead to a “bridge” of molten solder flowing onto the surface of the circuit board.Ensure proper cooling of the PCB circuit board with strong wind after soldering.
After cooling the circuit board, cut off the component leads.
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