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What is the difference between lead-free process and lead-free process? You can understand a form
Date：2021-12-10 14:43:00 Views：3511
Concept:
Lead free process: a basic concept of lead-free electronic assembly is that in the soldering process, whether manual soldering, immersion welding, wave soldering or reflow soldering, the solder used is lead-free solder (Pb feersoder). Lead free solder does not mean that 100% of the solder is lead free. In lead solder, lead exists as a basic element. In lead-free solder, the basic element does not contain lead.
Lead containing process: in the traditional soldering process of printed circuit board assembly, tin lead (Sn Pb) solder is generally used, in which lead exists and plays a role as a basic element of solder alloy.
difference:
Lead containing technology has a development history of more than 100 years. After the research of a large number of lead containing technology experts, it has good welding reliability and stability and mature production technology, which mainly depends on the characteristics of lead containing solder alloy.
Lead solder alloy has low melting point, low welding temperature and less thermal damage to electronic products; Lead solder alloy has small wetting angle and good weldability, and the possibility of "false welding" of product solder joint is small; The solder alloy has good toughness, and the vibration resistance of the formed solder joint is better than that of lead-free solder joint.
The melting point temperature of alternative alloys is higher than that of existing tin lead alloys. For example, according to the "tin silver copper" alloy which is likely to be widely accepted in the industry, the starting point is 217 ℃, which will greatly reduce the process window in the welding process.
| category | Lead free process characteristics | Lead process characteristics | |
| Solder alloy | Melting point temperature of solder alloy | High temperature 217 ℃ | Low temperature 183 ℃ |
| Solder solderability | difference | good | |
| Solder joint characteristics | Solder joint embrittlement | Good toughness of solder joint | |
| welding technology | Water cleaning process | Not recommended | have access to |
| Reflow soldering | The process window is small and it is difficult to adjust the temperature curve. Solder joint voids are difficult to eliminate. Bad tin on solder joint | The process window is large and the temperature curve is easy to adjust. Solder joint voids can be eliminated and solder joints can be coated with tin | |
| Wave soldering | The tin on the solder joint is not good, the cooling needs to be accelerated, and the detection frequency of tin bath alloy impurity content increases. It is possible that the production site needs detection instruments | The tin on the solder joint is good, and the detection frequency of the impurity content of the tin bath alloy is not large, so there is no need for on-site detection instruments | |
| Manual welding | Accelerated loss of soldering iron head | The loss of soldering iron head is small | |
| PCB requirements | board | The plate used when there is lead can be used, and it is better to use high Tg plate. Using high Tg plate, the plate cost increases by 10% ~ 15% | The sheet does not need to be changed |
| Pad treatment | Organic solderability protection (OSP), chemical nickel gold | Hot air leveling, organic solderability protection (OSP) and chemical nickel gold can also be used | |
| OSP features | Flat pad, high requirements for printing process, short PCB storage time and high requirements for plan. Impact on ICT testing | ||
| Characteristics of chemical nickel gold | There is the possibility of "black disk" | ||
| Heat resistance | High requirements | The requirements are not very high | |
| Weldability | High requirements | General requirements | |
| Solder joint inspection | appearance | The solder joint is rough and difficult to inspect | The solder joint is bright and easy to inspect |
| popcorn effect | Due to the increase of temperature, the moisture-proof sensitivity of many ICs will increase by one or two levels in lead-free welding. In other words, the moisture-proof control or treatment must be strengthened. This will have a serious impact on users with very small batch production. Because many small batch production users have a long inventory time of incoming materials. If the moisture-proof facilities in the inventory are not ideal, the problem of "popcorn" must be prevented by baking and dehumidification before assembly. Although baking can solve the problem of "popcorn", it will aggravate the oxidation of the welding end of the device and bring the difficulty of welding. It is feasible to bake in an inert environment, but this greatly increases the cost of equipment, consumables (inert gas) and management | The moisture-proof sensitivity of IC can be easily controlled as long as it is paid attention to and managed | |
| Monument phenomenon | It is more serious in lead-free technology. This is because the surface tension of lead-free alloy is strong. The principle of solution is the same as that of lead technology, in which the size of welding end and pad and the heat capacity at both ends of DFM controller are the most effective. Secondly, the temperature difference at both ends of the device can be reduced through process adjustment. It should be noted that although the principle remains unchanged, the lead-free process window will be smaller, so users must first ensure that the furnace they use has sufficient capacity, that is, good heating efficiency and stable air flow | It exists in the lead technology, but the lead process window will be wider and easy to solve | |
| Stomatal phenomenon | It is not easy to completely solve the problem in tin lead technology. After entering the lead-free technology, this problem will become more serious with the improvement of the surface tension of lead-free alloy. To eliminate the problem of "pores", three factors must be paid attention to; Solder paste characteristics (solder paste selection), DFM (device welding end structure, pad and template opening design) and reflow process (setting of temperature curve). The control principle is no different from that in lead containing technology, and the knowledge process window is smaller | The problem of "porosity" in tin lead technology is not easy to be completely solved | |
| Tin whisker growth | Without the restriction of lead, the growth rate is faster | Slow growth | |
| Electromigration | High occurrence frequency | Low occurrence frequency | |
