The reliability of lead-free welding interconnection is determined by these seven factors

Date：2021-09-15 18:24:24Views：1014

In the electronic manufacturing industry, the words RoHS and lead-free are the most seen. Many chips use lead-free materials, such as pins and tin balls, and slowly change from lead to lead-free. Reliability has become the focus of many people's attention. Unlike other lead-free related issues (such as alloy selection, process window, etc.), we often hear very different views on reliability. At the beginning, I heard many people say that lead-free is more reliable than tin lead, and some people say that tin lead is more reliable than lead-free. How should we judge at this time? In fact, it depends on the specific situation.

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. Depending on 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. Depending 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. Depending on PCB Laminate

According to the properties of laminates, some PCBs (especially large and complex thick circuit boards) may have higher failure rates such as delamination, lamination fracture, Cu crack and CaF (conductive anode wire whisker) failure due to high lead-free welding temperature. It also depends on the PCB surface coating. For example, it is observed that the joint between welding and Ni layer (from ENIG coating) is more likely to fracture than that between welding and Cu (such as OSP and silver immersion), especially under mechanical impact (such as drop test). In addition, in the drop test, lead-free welding will cause more PCB cracks.

4. Depending on 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. Depending 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 easy fracture of welding interconnection (and / or PCB).

6. Depending on thermomechanical load conditions

Under thermal cycling conditions, creep / fatigue interaction will lead to solder joint failure (i.e. microstructure coarsening / weakening, crack emergence and expansion) through damage accumulation effect. Creep stress rate is an important factor. The creep stress rate changes with the range 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 less thermal cycles than Sn Pb solder joint under "severe" conditions. Thermomechanical load depends on temperature range, component size and CTE mismatch between component and substrate.

For example, it is reported that on the same circuit board that has passed the thermal cycle test, the number of thermal cycles experienced by components with Cu lead frame in sac solder joint is higher than that of Sn Pb solder joint, while components with 42 alloy lead frame (with higher CTE mismatch of PCB) will fail in sac alloy solder joint earlier than Sn Pb solder joint. Also on the same circuit board, the number of thermal cycles of solder joints of 0402 ceramic chip devices in sac exceeds Sn Pb, while that of 2512 components is the opposite. As another example, many reports say that when the thermal cycle is between 0 ℃ and 100 ℃, the solder joint of 1206 ceramic resistor on FR4 fails later than Sn Pb in lead-free welding, while the trend is just the opposite when the temperature limit is - 40 ℃ and 150 ℃.

7. Depends on "acceleration factor"

This is also an interesting and closely related factor, but it will complicate the whole discussion 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. These factors are complex and interact with each other. For a detailed discussion, please refer to the latest book lead-free solder interconnect reliability.

As for the qualification tests that can be performed for lead-free devices, since lead-free processing requires higher reflow temperature, LSI uses high-quality materials that can withstand lead-free processing conditions and low-k and pad on I / O processes to qualify lead-free solutions. Qualification tests include but are not limited to: humidity resistance test (MRT), temperature cycle test (TC), temperature, humidity and bias test (THB) and high temperature storage test (HTS). In addition to component level testing, the LSI also collects secondary data to ensure that the components are in good quality when mounted on the PCB. Tin shall be qualified according to JEDEC ja210 standard, including temperature cycle test in the range of - 55oc to + 85oc, room temperature storage test and storage test in the temperature range of 55-60o / 85 and 90% relative humidity.