PIN connection is the most widely used connection because it is very steady. Unfortunately, not everyone is an expect at soldering. Every now and then, we can hear customers complaining about the failing rate was very high after the PINs were soldered on PCB. And I was always wondering if they did the soldering right. It was more than often that it was the soldering problem. It was not the quality problem of LCD screen itself. We felt it was our obligations to do some tests to help these who was lack of experiences.
In order to test the impact of the soldering temperature, we deliberately made it 350 degree Celsius far higher than the normal. The soldering iron head was placed at the distance of 4.0 mm from bottom of LCD screen.
It only took about 4.0 seconds that it began to melt the glue and then it bubbled. It would cool down a few seconds after the soldering iron head was removed and then it solidified slowly and shrank when it was completely cooled.
One interesting idea was that we located the PIN which was responsible for the missing segments of LCD screen and re-soldered it with over heated temperature. Pulled the PIN down a little, which would make it temperately connected in normal again. We could see it displaying the whole diagram for a while. When we were glad that we fixed the LCD screen, the old problem repeated again. It was easy to understand because the over heated temperature damaged the glue and turned the chemical composition into something else. Now the LCD screen was completely damaged and was unrepairable.
Let us try another experience. We soldered the perfect normal LCD screen with over heated temperature. It was as sure as the sun comes from east that the old missing segment problem recurred. I am going to remind everyone here don’t ever solder the PINs with over heated temperature and it will definitely cause the missing segment problem.
Let us hit the PINs with over heated temperature while we don’t use the UV glue to fasten the positions of the PINs. Wait. Is it possible not to use UV glue? Of course, it isn’t. It is never be able to fasten the PINs without UV glue. This was just an experience to explain what would happen in a hypothetical situation. Now we continued the experience. We put a 350 degree Celsius soldering iron head directly on PIN for 10 second and more until the polarizer turned yellow. To our surprised, the LCD screen was working perfectly fine. Now we could conclude that it was UV glue which was over heated that caused the missing segments and other problems. But we can’t make a LCD screen without it.
Let us test the dark green and colorless UV glue and see if the color has any effect on over heated temperature experience. When we used colorless of UV glue, a 350 degree Celsius soldering iron head was put directly on PIN and at the distance of 4.0 mm away from the bottom of LCD screen. It took about 6 to 8 seconds to see the bubbles. While it only took 4.0 seconds to see the bubbles when we adopt the dark green UV glue. Additionally, the colorless UV glue was harder than the dark green UV glue when it was dried. We can assume that the colorless UV glue is better and it is more high temperature resistance.
Finally, we make a few suggestions for welding LCD PINs.
1. The distance from the soldering iron head to the bottom of LCD screen should be at least 4.0 mmm and the temperature should below 260 degree Celsius. The time should be less than 4 seconds for the soldering iron head on PINs to avoid making the glue bubbled.
2.The soldering iron head should contact two adjacent PINs at the same time and make the heat transfer more even. It should form 30 degree angle with PINs because this will make contact area bigger and heat transfer faster. It will be a perfect heating condition if we can make the two adjacent PINs reach the same temperature at the same time.
3. It is forbidden to let the PINs bear any pressure while we are welding the PINs. It is going to prevent the PINs displacing because of the pressure from outside. It is best if we lay the front surface of LCD screen flat on a table.
4. It shouldn’t solder more than 3 times and wait at least 5 minutes before you next try.
5.The two PINs on the two ends bear a greater tension and they are more likely to be damaged because of the pressure. We strongly suggest that we add two more extra PINs which are not connected on two ends to lighten the burden if the products have a high quality requirement.