PCB Blog - Vibration test and impact test of PCB

Vibration test is an important part of PCBA test to assess the ability of products to withstand vibration environment (with different vibration levels), check and analyze the vibration resistance defects of products in design and manufacturing, so as to improve the PCBA manufacturing and ensure the reliability of products in use and transportation.

PCB vibration test on vibration table

The vibration test is carried out in the laboratory by simulating various vibration environments and fastening the samples on the special fixture of the PCB vibration table. The test can be divided into fixed frequency vibration tests called fixed frequency vibration tests; The other is the vibration test with logarithmic frequency conversion, which is called frequency conversion vibration test or scanning vibration test. The constant frequency vibration test is carried out according to the vibration test standard specified in the technical conditions of the product. The frequency, vibration acceleration, vibration direction (can be one, two or three directions) and vibration time are determined. When it is necessary to check whether the part material has fatigue damage under the continuous vibration stress, the vibration time can be extended.

The frequency conversion vibration experiment is a cycle from low frequency to high frequency, and then from high frequency to low frequency (its frequency is continuously changed in a logarithmic manner), which is called scanning once. The time and number of cycles, frequency range and vibration direction are specified in the technical conditions according to the use requirements of the product. During the scanning vibration test, observe whether the test sample has resonance phenomenon and the destructive effect on the product structure during resonance. Impact test is a test to test the ability of the product to withstand impulse impact and non-repeated impact during use and transportation, which is used to determine the adaptability of the product under mechanical impact and the firmness of its structure.

When the impact is applied, the kinetic energy transfer time to the product is the shortest. The magnitude of the impact force can be expressed in terms of impact acceleration, and it can also be expressed in terms of gravity acceleration g. The shorter the impact time and the greater the impact acceleration, the greater the impact on the product. When the waveform is different, it corresponds to different spectrum, and different spectrum will have different impact on the product. The duration describes the duration of impact action. Under the same conditions, the longer the duration, the greater the impact on the product. The simulated impact test in the laboratory is carried out by fastening the product on the special fixture of PCB impact test bench (not in working condition). The shock pulse generated by the shock machine can be terminal peak sawtooth wave, semi-normal wave and trapezoidal wave. The pulse peak acceleration and pulse width (duration) of various shock pulse waves used in the test are specified by relevant standards. During the test, impact the three mutually perpendicular axes of the sample in the positive and negative directions for three consecutive times, a total of 18 times. The free drop test is also the simplest impact test. It is generally used to test the strength of the packaging structure by raising the packaging to a height of 0.5~0.8 meters from the ground. Then let it fall freely and impact on the rigid cement floor. 

The impact position and times of the sample are specified according to the product technical requirements in the PCBA scheme.

1. Basic structure of reflow welding furnace

The typical infrared hot air reflow welding structure is shown in the figure. It is usually composed of more than five temperature zones. Each temperature zone is equipped with areal far-infrared heating and hot air heater. The temperature rise range of the first and second temperature zones is from room temperature to 150 degrees. The heating of the third and fourth temperature zones plays a heat preservation role, mainly to heat the SMA, so as to ensure that the SMA enters the welding temperature zone in a fully good state, The fifth temperature is the welding temperature zone. The SMA is cooled at normal temperature after being discharged.

2. Heater system

1) Heater

There are many types of heaters, which can be generally divided into two categories. One is infrared lamp, quartz tube heater, which can directly radiate heat, and is called sequential radiator; The other is alloy aluminum plate and stainless steel plate heater. The heater is cast in the plate, and the heat is first transferred to the plate surface through heat conduction. The tubular heater has the advantages of high working temperature, long radiation wave length and fast thermal response. However, due to the light generated during heating, different reflection effects are required for welding components of different colors, and it is also not conducive to matching with forced hot air. The plate heater has slow thermal response and slightly low efficiency. However, due to its large thermal inertia, it is beneficial to the heating of the heat seal through perforation, has little sensitivity to the color of the components to be welded, and has little shadow effect. In addition, it has strong structural integrity, and is also conducive to loading, unloading and maintenance. It also has obvious advantages over the former in terms of matching with thermocouples. Therefore, in current flow welding, almost all heaters are stainless steel heaters. Some manufacturers have coated their surfaces with infrared coatings to increase their infrared emission capability. The power of plate heater is 3~4kW for reflow welding of 400mm wide PCB, the power of each heater is 30~40kW, and the power of the whole machine is about 20kW after starting.

2) Control of wind speed and air volume

It is very important to realize the control of wind speed and air volume for the stable control of furnace temperature. The reflow soldering furnace of BTU can also realize the control of air pressure.

3) Transmission system

Nowadays, the conveying system of reflow soldering furnace generally adopts chain conveying. The width of the conveying chain can be adjusted mechanically or electrically. The PCB can be placed on the chain track, which can be connected for production, and can facilitate the double-sided welding of SMA. When purchasing reflow soldering, the running stability of the chain guide should be observed to avoid disturbing the generation of solder joints. Some guide rail materials do not have aging and temperature resistance treatment, and deformation occurs after working for a period of time. Whether the chain guide rail itself has a heating system is also a problem that cannot be ignored, because the guide rail also participates in heat dissipation, and will directly affect the temperature on the PCB edge. In addition, the heat resistance of the material of the guide rail itself should also be taken into account, otherwise the long-term operation at high temperature will cause rust and deformation. The parallelism of the chain guide rail is also a problem that cannot be ignored. Poor accuracy sometimes causes PCB to fall off in the furnace cavity.

4) Temperature control system

The temperature control system with furnace temperature test function can achieve high temperature control accuracy whether it uses a temperature control meter to control the furnace temperature or a computer to control the furnace temperature. The temperature control accuracy of lead-free reflow soldering furnace can generally reach ± 1 ℃. Infrared reflow soldering with the above basic functions can meet the needs of high-precision SMA products and lead-free soldering on the PCB.