The hottest microwave module assembly solder joint

Microwave module assembly solder joint and its reliability

Abstract: This paper introduces the application of microwave components and the importance of assembly and welding, puts forward the characteristics of solder joint in assembly and welding, analyzes the failure mechanism of solder joint in detail, expounds the influence of mechanical stress and thermal stress on solder joint failure, and finds effective measures to resist failure and fracture in welding process and solder joint design, so as to ensure the quality of solder joint, Improve the reliability of microwave components

key words: microwave components; Welding spot; Reliability

Introduction

microwave components are widely used in television broadcasting, satellite communication, relay communication, mobile communication, radar and many other fields. In recent years, with the development of phased array radar technology, a large number of applications in the radar field, because this is a new technology microwave components. The development of microwave module assembly technology is centered on improving quality and yield, shortening the production cycle, reducing costs, improving production efficiency and enhancing the adaptability of variety change. Microwave component assembly technology includes manual assembly, assembly line automatic assembly after power on and computer integrated automatic assembly. At present, microwave components used in the radar field are basically manual assembly

in the assembly and production process of microwave components mainly using manual welding, assembly faults account for more than 80% of the total faults. The outstanding performance of assembly faults is the quality of solder joints, which directly affects the reliability of microwave components composed of modules. High welding reliability and high qualification rate of solder joints are the deliberate goals in the assembly and production process of microwave components. The quality of solder joints should include two aspects: the design of solder joints and production control. Solder joint reliability is the life of microwave components, especially for aerospace products

I. solder joint characteristics of module assembly

solder joints of module assembly in microwave components have different characteristics from traditional integrated circuit solder joints:

(1) microwave components adopt many kinds of components, with a wide range of overall dimensions and weight distribution, precise structure and high dimensional accuracy. Most of them appear in the form of small modules, not standard SMT solder joints

(2) different modules in the microwave module have different connection methods, so there are many types of solder joints

(3) most of the modules in the microwave module are connected with the high-frequency printed board, and the high-frequency printed board is not allowed to drill welding holes. Welding can only be carried out on the surface of the printed board, and the bonding force of the welding points is weak

(4) the electrical properties of microwave components are sensitive to the deviation and inconsistency of parasitic parameters, size and structure, and the solder joints must be strictly controlled

II. Solder joint failure analysis

the purpose of studying solder joint failure is to provide basis for the design of welding process and solder joint, so as to improve the reliability of solder joint. From the solder joint failure in the service process of microwave components, it can be found that the solder joint that fails first is the module pin point. As shown in Figure 1, the solder joint between typical modules and high-frequency printed boards cracks and fails. These modules are non-standard SMT devices, and these modules have the characteristics of linear size and direct installation with the shell

the failure process of solder joint in the actual working process is generally: plastic deformation → crack initiation → crack propagation → failure. From the actual environmental conditions experienced by the solder joint, the internal stress is the root cause of solder joint failure, and the solder joint stress is composed of mechanical stress and alternating thermal stress

(1) the generation of alternating thermal stress is mainly because the device and installation shell are composed of different materials, which have different linear expansion coefficients. During the temperature cycle test, due to the alternating change of temperature, the expansion of the device and shell changes with the temperature. The device and shell are hard connections without buffer devices, and the welding point cannot expand and contract freely. The shell and device restrict each other, resulting in thermal stress at the welding point, Figure 2 is the module installation diagram

the thermal expansion mismatch of the small module is:

Δ L = ( α 2 — α 1 ) L Δ T

where, α 1 is the thermal expansion coefficient of the device; α 2 is the thermal expansion coefficient of the shell; L is the length of the device; Δ T is the temperature variation range. The shell of small module is generally made of stainless steel, and its expansion coefficient α 1 =0. one hundred and twelve × 10 - 4/c °, the shell is aluminum alloy, and its α 2 = 0. two hundred and thirty-eight × 10-4/c °, the difference of thermal expansion coefficient between the two is more than twice. Temperature variation range Δ The larger t is, the larger the mismatch is, and the greater the thermal stress is. The temperature change should not only consider the variation range of the ambient temperature, but also the different heat capacity, thermal conductivity and heat transfer coefficient between the device and the shell. When the temperature changes, the temperature change rate between the device and the shell is different, which aggravates the thermal mismatch phenomenon and increases the thermal stress. In addition, the periodic alternating change of temperature makes the thermal stress change periodically accordingly, Under the action of this periodic thermal stress, the welded joint will fracture

(2) the circuit dielectric board and small module in the microwave module are fixed on the shell with bolts, and the mechanical stress is mainly caused by the following aspects

a. the height of pad and small module pin should be 0 according to SMT specification 05～0. 1 mm, when the corrosion-resistant titanium and copper alloy used for small module pin marine equipment are lower than or higher than the plane of the dielectric plate pad, the pin deformation will produce mechanical stress during assembly

b. improper welding process, using the sequence of welding first and then bolt tightening, resulting in mechanical stress at the pins

c. although the assembly adopts the process of tightening bolts first and then welding pins, if the bolt tightening torque is insufficient when installing bolts first, and the secondary bolt tightening operation is carried out after welding, residual mechanical stress will be generated at the pins, and the loosening of bolts will also produce residual mechanical stress

d. the installation surface of the shell and small module is non ideal plane due to machining error, which will produce elastic deformation during installation and generate residual mechanical stress at the welding point

e. in the accelerated vibration test, due to the effect of acceleration, the fixed bolt and shell mounting surface are elastically deformed, resulting in mechanical stress at the welding point, and this deformation is greater

according to the mechanical analysis:

Δ L = km a

where, Δ L is the elastic deformation caused by acceleration during vibration; M is the mass of small module; A is the vibrator speed; K is a constant (a constant related to the size and strength of the bolt). It can be seen from the above formula that the larger the mass of the small module is, the faster the vibrator speed is, and this deformation is greater. The above analysis considers the mounting surface of the microwave module shell as an absolute rigid body. When the rigidity of the mounting surface of the shell is insufficient, the elastic deformation is greater, and the stress at the solder joint is greater

due to the large linear size of the small module, which is 1 ~ 2 orders of magnitude larger than the general SMT device, the high quality and non buffer assembly and welding form, greater thermal stress and mechanical stress are generated at the welding position. The combined effect of thermal stress and mechanical stress produces fatigue failure at the welding leg, resulting in fracture, which is the main reason for the fracture of the welding joint of the small module

according to the mechanical analysis:

Δ L = km a

III. method of resistance to failure and fracture of welded joints

periodic thermal stress and mechanical stress of welded joints lead to fatigue fracture of welded joints. The performance of resistance to fatigue fracture of welded joints is not only related to the size of periodic stress, but also related to the properties of welded joints (such as welding materials, joint structure, welding surface characteristics, welding process), etc

1. Improve the structural rigidity and flatness of the installation surface. The module has large mass and large vibration inertia. If the rigidity and flatness of the installation base are poor, the fixation is unreliable, and mechanical stress is easy to occur during installation; In the process of vibration, it is easy to produce elastic deformation and periodic stress in the solder joint. Because the bonding force between the solder joint and the microstrip line is stronger than the adhesion between the microstrip line and the substrate, the microstrip line is torn

2. Improve the assembly and welding sequence of small modules. The assembly and welding sequence of small modules has an important impact on the stress of welding points. It is strictly forbidden to adopt the process of welding pins first and then fastening bolts. Using torque driver, the small modules are assembled with constant torque to improve the connection reliability of each small module

3. Standardization and stabilization of welding process parameters. Welding process also plays an important role in the quality of welding joints. It mainly includes solder composition, printing solder quantity, welding temperature, welding time, surface treatment method of welding surface, etc. In addition to the above conditions, the cleanliness of the welding surface is also very important. After welding, there should be a good anti oxidation storage environment. Nitrogen can be used to protect the dehumidification box or vacuum storage to ensure the quality of the welding point

4. Standardize solder joint design

(1) strictly control the gap size in the direction of pad and pin height, preferably within 0 About 1 mm

(2) the pad size is larger than the pin size, and generally one side is 0 About 2 mm

(3) in order to reduce the influence of thermal stress and mechanical stress, an Ω buffer link is added between the pins of each small module and the pad without affecting the electrical performance

IV. conclusion

through the analysis and improvement of the assembly and welding process and design of the microwave module module, the environmental adaptability and reliability of the microwave module have been greatly improved, so as to meet the reliability requirements of the whole system. The design method and process technology can be used for reference in the research of other types of solder joints. (end)