The use of lead-free products will make the product repair rate rise
"Do you know the problem of lead free," said a senior reliability specialist in general motors. No one can answer such a simple question: if I make the quality of the lead free products and the use of tin lead products, they work as long as the same time?"
The question is simple, and the answer is not complicated. After many years of research and experiments, researchers have put the impact of lead-free products, long-term reliability of the factors attributed to the following 5 aspects.
The tin whisker
Kirkendall - empty
The mechanical vibration of lead-free solder
The thermal cycle of lead-free solder
The conductive anodic filament (CAF)
tin whisker
Tin is to be grown from the tin coating of components and connectors, if these conductive tin is too long, it may be connected to other lines, and lead to electrical short circuit. By using the 90Sn10Pb coating on the device pin has effectively eliminated the problem of tin whisker, but now the problem has come out again.
Some companies produce a large number of lead-free products with pure tin, which has been found to be a problem in the process of the use of tin, and in less than 2 years, there will be a failure. Although the actual rate of fault is unclear, but evidence shows that tin whisker fault does not make the repair product overall rate increase. Due to the fact that tin plating is the last three or four years in the thin pin device, the effect of the tin on the long-term reliability remains to be seen.
If you are not in the design of products that may cause personal injury, components and connectors should first consider whether they have the following characteristics.
The pin spacing is less than 1mm (some companies use spacing less than 0.3mm)
Can be made of metal (such as holes or crystal oscillator)
- by the connection point of the backlog (e.g. bending circuit)
- weld (e.g. electrolytic capacitor)
Once the key device, you should ask the manufacturer to provide certification device to test the proposed iNEMI or JEDEC standard based on JESD22A121. The industry of fault definition is almost empty, so you may need to put forward their own standards. The standard can be both absolute values, such as whisker length cannot be the largest of more than 25, 50 or 75 m, can also be a relative value (such as the minimum distance of 1/3 or 1/2).
Kirkendall empty
Between the two kinds of similar material, the hole diffusion rate generated by the different Kirkendall is called empty, the empty mechanism exists in SnPb and lead-free solder. In a recent experiment, SnPb solder will be produced in the hole is serious. But in the process of surface mount technology over the past 30 years of use, not because of the Kirkendall hole lead to product failure report.
In lead-free solder, Kirkendall cavity is caused by some unknown factors, seems to make the problem more serious, especially in the long term under high temperature conditions.
Mechanical vibration
Even if you haven't seen Kirkendall empty, ready for the decline of product performance or mechanical shock caused by falling or. Studies have shown that is applied in the vibration, drop or circuit board is bent, SAC (SnAgCu) solder alloy failure load is less than half of SnPb alloy. The circuit board for this performance loss seems to be the combined effect of several factors, including fragile intermetallic compound, due to higher reflow welding temperature and lead to downgrade due to sac than SnPb more hard and passed the greater stress.
This is a long-term reliability problems, or from the beginning is a quality problem? Lead-free manufacturers of portable electronic products has been implemented in the years, not because of drop causes the rework rate of increase in the report. Some companies focus on how to better control the manufacturing environment, especially the maximum allowable tension decreases slightly from 1000, reduced to 750 or 500 rib variable micro fin variable.
thermal cycle
In certain cases, the thermal performance of SnPb material is better than unleaded, should consider the following situations.
] on solder joint fatigue highly sensitive elements (such as large on-chip resistors, ceramic package BGA, no pin ceramic substrate, unfilled chip scale package).
The maximum node temperature above 80 degrees Celsius, the dwell time more than 4 hours (dwell time decreases with the increased temperature).
] every day at least one thermal cycle, the expected service life of at least 10 years service life and decreases with the increase of cyclic frequency).
If you are in the situation mentioned above, the need for accelerated life test of existing node reliability model of lead-free solder is determined based on risk. Open the model more reliable, Amkor and Germany's Fraunhofer Institute has made a lot of work.
The formation of CAF
Conductive anodic filament (CAF) is made of copper wire along the glass fiber and resin interface formed by the migration and creates internal electrical short circuit between adjacent conductors, in the design of high density circuit board is a serious problem, the higher reflow temperature will lead to the problems are more likely to occur. Some large PCB panel makers in the use of lead-free reflow soldering, will not be able to meet the CAF requirements of users.