PCB Technical - The management process of lead-free soldering of circuit boards

The management process of lead-free soldering of circuit boards

1. Avoid mixing
During the transition period, the original lead wave soldering and the newly established lead-free wave soldering cannot be mixed. As for the stock parts that have not been consumed, and the leads are still the old tin-lead electroplating film, they should be soldered with lead. Otherwise, when the lead leads pass through the lead-free tin pool, it will inevitably cause lead contamination of the solder and also Cause lead contamination of other assembled boards. This may not only violate ROHS regulations, but also a small amount of lead contamination will cause insufficient solder joint strength. Therefore, when the two lines coexist, how to prevent mixing is indeed a sloppy priority.

Second, choose a new machine
As the heat of lead-free soldering increases greatly, it will inevitably cause serious damage to plates and parts. In addition, most PCB boards require two strong heat welding of Reflow and wave, and the disaster is particularly devastating. In addition, most industry players do not know much about lead-free heat, and they have to do wave soldering for some solder joint defects. The consequences are even more disastrous. Therefore, in order to avoid bad luck and turn good luck, we must continue to absorb new knowledge and use new methods in order to achieve the mission and smooth mass production.

The first is the establishment of lead-free wave soldering capacity. It is recommended to purchase a newly designed lead-free wave soldering machine instead of cleaning (using pure tin) or modifying the old machine to reduce annoying lead pollution. The structure of the wave soldering machine is relatively simple. Because there is no (ReflOw machine) hot air temperature that is easy to change and the difficulty of correct compensation in an instant, there is no need to consider various famous brands from the United States, Japan or Europe. In fact, Taiwan's self-produced models are already very useful. For example, within NT$1 million, the lead-free wave soldering unit launched by Song Yi Electric has a very good reputation.

Three, the operating parameters of the circuit board welding management process
From the previous soldering temperature curve of SAC305, it can be seen that when the bottom surface of the soldering board of the circuit board touches the tin wave, the traversal of the front surge is about 1-2 seconds, and the flat wave is about 2-3 seconds, and the total is about 4-5 seconds. It takes 1-2 seconds longer than lead wave soldering. If a special carrier (Special Pallet) has been adopted and only a local welder is exposed at the bottom of the board, it can be completed with only a single wave under the exclusive contact of concentrated tin heat. Into the plug welding and paste welding. For those who pay attention to the amount of tin filled holes (at least 75%) or the carrier is very thick, the welding can be completed by using the previous surge time alone (3 to 4 seconds). Generally, the simple board surface can also be welded with the flat wave at the back alone. In this way, the two waves before and after are used separately, which will reduce the damage caused by the two heat shocks.

It should be noted that between preheating to the tin wave, the surface temperature of the circuit board should not drop too much (must be less than 3 degree Celsius), so as to avoid cold welding with insufficient heat, it is best to install an additional 22O degree Celsius or more between the front wave Assisted by hot air, the current new machines have taken into account the demand for such lead-free characteristics. And in order to prevent the side of the tin pool from cooling and the whole board temperature is not uniform, the new lead-free wave soldering machine also adds a separate heat preservation pool outside the main pool to ensure that the temperature of the main pool does not drop instantaneously.

Once the preheating of the board surface or the pins outside the hole is not enough, the soldering temperature of the local contact will be reduced at the moment of entering the tin wave. Moreover, the viscosity will increase when the tin temperature drops, which will cause a bridge and short circuit between adjacent pins. Long-term research in the industry has found that under the fixed factors of PCB surface treatment, insufficient preheating accounts for 58% of the various factors that lead to insufficient tin in the hole, followed by soldering temperature, contact time, and flux brand. Minor factor.

There are four main parameters of wave soldering, namely: flux, preheating, soldering temperature, and contact time. Due to the different board area, the number of parts and the different sizes, etc., will affect the performance of wave soldering. Therefore, when replacing the board to be welded, it is necessary to test one piece in advance according to the circumstances, and then make the right experience. Each parameter is fine-tuned. The following are the descriptions of the main operating parameters:

(1) Flux
As far as the solderability of lead-free wave soldering and OSP film is concerned, the F1uX brand is very different, and you must carefully compare and evaluate it. For various current products or future water-based additives (VOC, waterbase), spray type is the best choice. The spraying amount is preferably 380-580mg/dm or 45-50ml/min, to achieve atomization but not bouncing. This can also avoid excessive liquid collection on the front or rear edges of the board, and reduce the color difference and uncleanness of the board surface after drying. The distribution of flux on the board surface can be tested and observed using thick cardboard.

(Two), preheating (Prchcat)
This station can increase the temperature of the board and parts to reduce the soldering effect of the instantaneous cooling of the tin wave, and provide energy to assist the flux to carry out the chemical reaction of rust removal, and then all the oxides on the surface of the lead and the solder pad Clear. At the same time, it can also drive away the solvent or water to suppress the splashing of tin when entering the wave, and reduce the occurrence of bad solder balls (Solder Ba11ing). The method to check whether the preheating is compliant is to use the temperature sensing line to measure the temperature of the upper board. The range is controlled between 90 and 130 degree Celsius according to the size of the board and the number of parts, and 110 degree Celsius is generally appropriate. For those adopting special carriers (Pallets), the board surface temperature drops to an average of about 70-80°C.

(3) Solder Temp
Although the circuit board lead-free solder can choose SAC305, SAC3807 with a melting point of 217 degree Celsius, or SCN (Ni0.02-0.05bywt) with a melting point of 227 degree Celsius, both types of solder can be adjusted to the peak temperature of 260-270 degree Celsius, and should Check the amount of dissolved copper in the pool every two weeks (0.9 g/1 is the upper limit), and confirm that its viscosity will not increase too much due to the increase in mp, thereby reducing the occurrence of short circuits and bridging.

(4) Contact Time
Refers to the numerous solder joints on the bottom surface of the board. In terms of the total contact time of a certain fixed point through two tin waves, the dip time of lead-free solder (that is, the growth time of IMC) is slower on average than those with lead Therefore, the contact time during operation must be 1 to 2 seconds longer. That is to say, the total should be controlled between 3 and 5 seconds, and if there are many pieces of the big board, it will be slightly extended according to the actual situation. Of course, this "contact time" dominates the walking speed of the conveyor chain, so it can be reversed from the speed adjustment to find out the short contact time. Generally, if the length of the wave soldering machine is 3.6 m, if the set contact time is 3-4 seconds, the overall travel progress should be between 1.0 and 1.2m/min.