Factors affecting the reliability of lead-free solder interconnects

Date：2022-03-15 18:25:21Views：837

"Lead free" here refers to Wuxi welding line. Lead free tin wire is a product of welding wire. Tin wire can be divided into lead-free tin wire and lead-free tin wire, both of which are used for the welding of circuit board. Lead free solder wire is also called environmental protection tin wire. Lead free solder wire is also called environmental protection tin wire. Its main components are tin (SN), silver (Ag), copper (Cu), and the rest include trace lead (PB), mercury (Hg), cadmium (CD), polybrominated biphenyls (PBBs), etc. It is characterized by good weldability, good wetting performance, uniform distribution of rosin in the line, good continuity, no odor, less smoke, no splash during welding, no toxic volatile gas, and a little yellow on the surface.

The reliability of lead-free welding interconnection is a very complex problem, which depends on many factors. We briefly list the following seven factors:

1. Depends on the welding alloy. For reflow soldering, the "mainstream" lead-free welding alloy is Sn Ag Cu (SAC), while wave soldering may be sac or Sn Cu. Sac alloy and Sn Cu alloy have different reliability properties.

2. Depends on process conditions. For large complex circuit boards, the welding temperature is usually 260 (c), which may have a negative impact on the reliability of PCB and components, but it has little impact on small circuit boards, because the maximum reflow temperature may be relatively low.

3. Depends on PCB Laminate. Some PCBs (especially large and complex thick circuit boards) may cause delamination, lamination fracture, Cu crack, CAF (conductive anode wire whisker), failure and other failure rates to increase due to the high lead-free welding temperature according to the properties of the laminate. It also depends on the PCB surface coating. For example, it is observed that the bonding between welding and Ni layer (from ENIG coating) is better than that between welding and Cu (e.g. the joint between OSP and silver impregnation is more likely to break, especially under mechanical impact (e.g. in drop test). In addition, in the drop test, lead-free welding will cause more PCB cracks.

4. It depends on the components. Some components, such as plastic encapsulated components and electrolytic capacitors, are more affected by the increased welding temperature than other factors. Secondly, tin wire is another reliability problem that the components with fine spacing in high-end products with long service life pay more attention to. In addition, the high modulus of sac alloy will also bring greater pressure to components and problems to components with low k dielectric coefficient, which are usually more prone to failure.

5. Depends on the acceleration factor. This is also an interesting and closely related factor, but it will make the whole discussion much more complicated because different alloys (such as sac and Sn Pb) have different acceleration coefficients. Therefore, the reliability of lead-free solder interconnection depends on many factors.

6. Depends on thermomechanical load conditions. Under thermal cycling conditions, Creep / fatigue interaction will lead to solder joint failure through damage accumulation effect (i.e. microstructure coarsening / weakening, crack emergence and expansion, creep stress rate is an important factor. The creep stress rate changes with the amplitude of thermomechanical load on the solder joint, so that sac solder joint can withstand more thermal cycles than Sn Pb solder joint under "relatively mild" conditions, but Under "severe" conditions, Sn Pb solder joints bear less thermal cycle than Sn Pb solder joints. The thermomechanical load depends on the temperature range, component size and CTE mismatch between components and substrate.

For example, the number of solder joints with Cu lead on the same circuit board with thermal cycle shall be higher than that of Sn lead on the same circuit board, Components with 42 alloy lead frame (the CTE mismatch of its PCB is higher, and the failure will occur in sac alloy solder joint earlier than Sn Pb solder joint. Also on the same circuit board, the solder joint of 0402 ceramic chip device passes through more thermal cycles in sac than Sn Pb, while the solder joint of 2512 component is on the contrary. For another example, many reports say that when the thermal cycle is between 0 ℃ and 100 ℃, the solder joint of 1206 ceramic resistor on FR4 The time of failure in lead-free welding is later than Sn Pb, while the trend is just the opposite when the temperature limit is - 40 ℃ and 150 ℃.

7. Depends on mechanical load conditions. The high stress rate sensitivity of sac alloy requires more attention to the reliability of lead-free welding interface under mechanical impact (such as drop, bending, etc.). Under high stress rate, excessive stress will lead to welding interconnection (and / or PCB, easy fracture).

The above is the content related to the reliability of lead-free welding interconnection compiled by the core detection team. I hope it will be helpful to you. Chuangxin testing is a professional testing organization for electronic components. At present, it mainly provides integrated circuit testing services such as capacitance, resistance, connector, MCU, CPLD, FPGA, DSP and so on. Specializing in functional testing of electronic components, incoming appearance testing of electronic components, anatomical testing of electronic components, acetone testing, X-ray scanning testing of electronic components and RoHS component analysis testing. Welcome to call, we will serve you wholeheartedly!