Reliability test of common welding products

Date：2021-11-22 11:50:00Views：1074

When selecting solder varieties, we should not only care about the mechanical properties of the solder itself, but also care about the reliability of the welding formed by the solder. In fact, the mechanical properties of solder are not completely equal to those of welding, especially for lead-free welding. The welding formed by solder forms MC at the joint between solder and copper pad. For tin lead solder, the mechanical strength of MC is greater than that of solder itself. When impacted by external force, the fracture usually passes through the solder itself. At this time, large external force impact energy is required. The lead-free welding is different. Under high-speed impact, the fracture will appear at IMC, and with the increase of impact speed, the probability of fracture at IMC will increase, and only low external force impact energy is required. Therefore, people pay more and more attention to the welding test of lead-free solder, which is usually used to evaluate the mechanical properties of solder. The most typical practices are as follows.

Board level test

Assemble the solder to be evaluated into a test board (usually do a comparative test of different solder), and then select the test method as needed

1. High and low temperature impact method

The method of high and low temperature impact method can be carried out according to the following standards:

● ipc-sm-785 guide for accelerated reliability test of surface mounted weldments;

● ipc-9701 accelerated reliability test method and qualification requirements for surface mounted weldments;

● IPC-TM-650 Manual of experimental methods

The specific method is to place the welded sample plate in a special high and low temperature test chamber. Ipc-9701 standard sets performance test methods under five test conditions, which are planned to be divided into five different grades from low required T100 ℃ to strict required T180 ℃ C. The greater the temperature difference, the worse the conditions, that is, the higher the requirements for welding reliability.

In each standard, stay at low temperature and high temperature for 30min respectively, and each change is a cycle. Usually, a test needs to be conducted for 2000 ~ 3000 cycles, and then evaluated by visual inspection or test according to ipc-tm6650 standard. The temperature difference of 5 different grades is:

● Tc1: 0 ℃ / 30min ←→ 100 ℃, t is 100 ℃, recommended reference;

● TC2: - 25 ℃ / 30min ←→ 100 / 30min, t is 125 ℃;

● TC3: - 40 / 30min ←→ 125 / 30min, t is 165 ℃;

● TC4: - 55 / 30min ←→ 100C / 30min, t is 155 ℃;

● TC5: - 55 / 30min ←→ 125 ℃ / 30min, t is 180 ℃

The test shall be conducted according to the following requirements during the whole test:

Determine the cracking rate every 100 cycles;

Peel strength is tested every 500 cycles.

The above test is conducted by imitating the working state of electronic products, that is, due to its own resistance, the welding will generate heat when there is current, but the welding will return to room temperature when it is not working. From high temperature to room temperature means that the welding will undergo high and low temperature changes. In some parts of automotive electronics, the welding temperature will reach more than 80 ℃, If the ambient temperature changes alternately, the welding temperature rise will be higher. Therefore, the test method has strong practicability, but the test cycle is long and the cost is high. Full preparations should be made before the test.

2. High temperature aging method

High temperature aging method is a commonly used test method, which is simple and reliable. Generally, the performance will decline after welding aging, and the decline degree of welding performance of different solders is different. The specific method is to put the sample in the oven for aging. The oven temperature can be 150 / 180 / 200 ℃, and the time can be 200 / 300 / 400H. Then test the welding strength and select the good solder through comparison.

3. Drop test

Drop test is one of the traditional methods to evaluate welding reliability. It is simple and low-cost. At present, it is mainly used to evaluate electronic products that are easy to fall to the ground, such as mobile phones and other portable products. Because such products often fall to the ground when used, and electrical faults will occur, including the cracking of the welding between components and circuit boards. In addition, drop test can also be used for comparative test. For example, when drop test is conducted for lead-free WLCSP components, the reliability of welding with underfill and without underfill can be compared. It is usually found that lead-free WLCSP components with underfill have good reliability. The drop test can refer to GB2423 and JEDEC jesd22-b111 standards.

4. Welding high speed impact test

Although people pay attention to the brittle fracture caused by lead-free solder, the early shear and tensile test methods make the brittle fracture failure very uncommon, not because the brittle fracture will not occur, but because the early test system can not provide a stable force to prove the existence of this failure mode. In recent years, people have developed a welding high-speed impact test system, which makes the test simple.

Welding means by heating or pressurizing, or both. And a way to achieve atomic bonding of weldments with or without filler materials. At present, there are three welding methods: fusion welding, pressure welding and brazing. Welding has the advantages of metal saving, high production efficiency, good compactness, high strength, easy operation, and easy to realize mechanized and automatic production. It is widely used in machinery manufacturing, boiler, chemical industry, petroleum and natural gas pipeline, civil construction and large steel structure manufacturing. It has replaced the traditional riveting process.

In summary, the reliability of welding is guaranteed through welding evaluation. In short, first determine the welding process through welding evaluation, and then ensure that the welding personnel can weld the welds that meet the quality requirements through welder qualification evaluation. The last is the reasonable welding process to ensure that the corresponding requirements are met.