GB/T 2424.21-1985 Basic environmental testing procedures for electrical and electronic products Guidelines for solderability tests using the wetting and weighing method GB/T2424.21-1985 Standard download decompression password: www.bzxz.net

National Standard of the People's Republic of China
Basic environmental testing proceduresfor electric and electronic productsGuidance on solderability testing by the wetting balance method
Definition of wettability test
UDC 621.3
:620.179.2
GB 2424.21-85
When the test sample is immersed in molten solder, the wetting balance method can express the vertical force acting on the test sample as a function of time (see Figure 1), and derive the wettability of the test sample from the above functional relationship in the form of the time to reach a given degree of wetting or the degree of wetting reached within a given time. Force F
Time!
The standard for wettability generally requires that if ten points on the force-time curve meet the specified values, the points and values ​​recommended by this guideline can be used.
Published by the National Bureau of Standards on May 15, 1985
Implemented on February 1, 1986
GB 2424.21--85
If reproducible and quantitative results are required, the test equipment must comply with the requirements of GB2423.32-85 "Weldability Test Method by Lubrication and Weighing Method". These requirements and the identification methods based on these requirements are given in this guideline. 2 Shape of test sample
The test sample can have any shape, but the immersion part has a uniform cross-sectional area, such as a rectangular or circular test sample. This can simplify the analysis of the curve and the calculation of the force. In order to reduce the calculation error, when the test sample is immersed, the angle between its test surface and the plumb line should be within ±15°. The cross section of the immersion end of the test sample should be 90° to the axis and should not have burrs. This test can be used for test specimens such as chip capacitors or printed circuit boards that have large areas that cannot be wetted by solder. However, these areas may have an impact on the force-time curve. Therefore, the current test standards are mainly applicable to the leads of components and devices, and the entire cross-section of the leads of such components and devices can be wetted by solder around the periphery. 3 Preparation of test specimens
It is very important to use standard procedures for solder dipping and removing excess solder for the test specimens in order to prevent the curve measured during the test from being disturbed by the effects of flux volatilization or flux dripping. 4 Characteristics of test equipment
4.1 Recording device
4.1.1 Zero point adjustment
During the test cycle, when changing from non-wetting to wetting, the force acting on the test specimen changes direction. In some cases, buoyancy can cause a considerable vertical deviation of the wetting curve. In order to record the entire wetting curve under the highest sensitivity conditions, it is necessary to operate the curve recorder so that the zero position is at the appropriate position on the recording paper to keep the entire curve on the recording paper. 4.1.2 Response time
The response time must be small enough to ensure that the recording device can accurately reproduce the rapid changes in wetting force, especially at the beginning of wetting. Although it should be infinitesimal in theory, practice has shown that a maximum response time of less than 0.3 s can meet the test requirements. The procedure for measuring the response time and zero point stability of the test equipment is as follows. When measuring, a code of known weight and a test fixture of a shape suitable for carrying the code are used. The weight of the code should be sufficient to give the deflection of the recording pen from the middle zero point to the full scale. a. With the test fixture installed, adjust the recorder to zero position; b. Start plotting at the highest speed gear,
c: Place the code on the fixture,
d. Remove the code after 2 or 3 seconds, leaving the recorder still running freely; e. After another 2 or 3 seconds, put the code back on the fixture, f. Repeat the procedures d to e at least 5 to 6 times and then turn off the recorder. The curve obtained on the recording paper gives the sensitivity of the device in the selected range, the response time of the recording pen and the consistency of its return to zero.
4.1.3 Sensitivity adjustment
In order to test test specimens of different sizes, a sensitivity range adjustment device is required. This purpose can be conveniently achieved by using a curve recorder with a measuring amplifier with adjustable amplification. If these full-scale forces are between 20 mN and 1 mN (equivalent to a mass of 2 g to 100 mg), they can accommodate test specimens with circumferences between 20 mm and 1 mm. 4.1.4 Plotting speed
In order to be able to have sufficient discrimination of the inflection points in the force-time curve, a minimum plotting speed of 10 mm/s is required. 4.2 Scale system
4.2.1 Spring stiffness
The scale system is used to measure the displacement of the spring assembly caused by the force acting on the test specimen. This displacement causes a change in the depth of the test specimen immersed in the solder and a corresponding change in the buoyancy. Therefore, the spring system is required to have sufficient stiffness so that the deflection of the spring and the resulting change in buoyancy during the test can be ignored compared with other forces in the measurement. 4.2.2 Noise level
In the most sensitive test range, the electrical and mechanical noise level in the scale and amplifier system should not exceed 10% of the signal level. 4.3 Soldering bath
The soldering bath should have a large enough thermal capacity to maintain the test temperature according to the specified accuracy requirements, and the test sample should be far enough away from the bath wall so that the force acting on the test sample is not affected by the curved liquid surface at the edge of the solder surface. The bath temperature of 235°C was selected to improve the discrimination of the test. Some coatings, such as bright tin or gold plating, have a significant change in dissolution rate between 235°C and 250°C in a solder composed of 60% tin and 40% lead. In this case, the relevant standards stipulate that a bath temperature of 250°C can be used for testing. 4.4 Lifting mechanism and control of the tank
4.4.1 Immersion depth
The immersion depth of the test sample in the molten solder must be specified and must meet the following conditions: a. During the wetting process, the rising solder meniscus must cross the area of ​​interest and at the same time, the test sample must maintain a certain distance from the bottom of the solder tank. If necessary, the end of the test sample needs to be trimmed. b. The area crossed by the meniscus should preferably have a uniform cross-sectional area. c. The adjustment error of the immersion depth should be within ±0.2 mm. Note: The deeper the immersion, the greater the deviation of the buoyancy line from the center zero position. Even in the case of complete wetting, the final signal may still remain above the initial equilibrium point. The deeper the immersion, the larger the effective interface for heat transfer from the solder to the test sample, so the wetting process is less delayed by the heat transfer effect. 4.4.2 Immersion speed
For standard operating methods, the immersion speed should be controlled in the range of 16 to 25 mm/s. Too high a speed may produce shock waves in the solder bath, which will interfere with the force measurement, while too slow a speed will cause the solder bath to be moving during the important initial stage of meniscus rise. 4.4.3 Collapse Duration
It is not usually allowed to solder test specimens for more than 10 seconds. However, for test specimens that are difficult to solder or have a large heat capacity, a dwell time of 10 seconds is required to collect sufficient information. For small test specimens (such as leads), a dwell time of 5 seconds is usually sufficient. Comparing the force values ​​recorded at the beginning of the test cycle with the force values ​​recorded at the end of the dwell time can provide information on the stability of the interface between the solder and the test specimen (see 6.1.3). 5 Typical force-time curves
On the force-time curve, the force acting on the test specimen, the upward part of the curve indicates non-wetting. We use positive values ​​to represent wetting; the downward part of the curve represents wetting, which we use negative values. The dotted line represents the situation at the beginning of the test cycle, with the weight of the test sample offset, and the horizontal solid line represents the buoyancy deviation when the wetting force is equal to zero.
The buoyancy of the test sample can be calculated from the volume of its submerged part and the density of the molten solder it displaces. At the specified test temperature of 235℃, the density value for molten solder of 60% tin and 40% lead is 8000kg/m3. A typical force-time curve is shown in Figure 2. 618
Complete non-wetting,
Poor wetting
Good wetting
Fast and limited heat
dissipation
Delayed wetting
6 Parameters measured from the force-time curve
GB 2424.21—-85
Slow wetting
Floating test
Test sample
Extremely fast wetting
Unstable wetting
6.1. Selection of test standards Www.bzxZ.net
One of the advantages of the wetting weighing method solderability test is that the entire wetting process is checked. When judging whether the test requirements are met, the official adopts one or more of the parameters listed in Article 6.4 of GB2423.32-85 "Wetting weighing method solderability test method". 6.1.1 Time to start wetting
At point A (Figure 1), the soldering process changes from a non-wetting state to the point where the meniscus of the solder begins to rise from the solder surface in the solder tank. Therefore, the time interval between t. and t. is the time required for the wetting start stage. For components assembled using a batch soldering procedure, this time is about 1 to 2.5 seconds, depending on the type of flux and the thermal characteristics of the test sample. 619
6.1.2 Wetting process
GB 2424.21—85
The solderability of the test sample is evaluated by the fraction of the actual wetting force achieved within the specified time as a percentage of the theoretical wetting force or the reference wetting force.
When the relevant standards do not specify, it is recommended to achieve 35% of the theoretical wetting force or the reference wetting force within 3 seconds when using an inactive flux.
Note: The measured wetting force includes the buoyancy offset. The influence of buoyancy should be eliminated first during the evaluation. 6.1.3 Wetting stability
After the maximum force value at point B (Figure 1) is obtained, the meniscus can remain stable and the force value no longer changes. However, this stability can be destroyed by the reaction between the test sample and the solder, causing the surface of the test sample to be dissolved by the solder or forming a layer of reaction products at the interface. In addition, the remaining flux may volatilize or decompose or migrate to the surface of the material. These effects may lead to a decrease in wetting force, making the wetting force at point C less than that at point B at the end of the test cycle. This instability is undesirable. For a test cycle of F5 to 10s, we recommend that the value of the wetting force at point C/the wetting force at point B should exceed 0.8, which limits the allowable change limit of the theoretical contact angle within this cycle to less than 10° (except for the initial value less than 20°). As mentioned in 1, it should be noted that the change in force may be caused by factors other than the actual contact angle. 6.2 Theoretical wetting force
In order to have a wetting force standard that is independent of the wetting properties of the test sample, the actual measured wetting force can be compared with the theoretical wetting force obtained by calculation after eliminating the buoyancy effect. The theoretical wetting force F (mN) can be calculated using the following formula: F = -0.4L
武: L-
The perimeter of the unbroken part of the test sample, mm. a. The theoretical wetting force acts on the plane of the test sample surface (i.e. the contact angle is 0). b. At 235°C, the surface tension constant corresponding to the specified flux and solder is 0.4N/m (=0.4J/m2). c. For this calculation, the density of molten solder at 235°C is approximately 8000kg/m3. Additional notes:
This standard was proposed by the National Technical Committee for Environmental Technology Standardization of Electrical and Electronic Products. This standard was drafted by the National Standard Compilation Working Group for Wetting and Weighing Method Solderability Test of the National Technical Committee for Environmental Technology Standardization of Electrical and Electronic Products.
The main drafters of this standard are Zhou Xincai, Wang Xiuqing, Lu Zhengji, Pang Huaixin, Wang Xiping, He Chengshan, etc. 620
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