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1Glass package components

This type of component is glass encapsulated, generally weighs 0.05 g, and has a diameter of 1.85 mm to 2.02 mm. The glass bulb is sintered at a high temperature of 600 ℃, and then forced to cool under cold nitrogen. The outside of the glass glaze is baked enamel, which combines very well with the glass glaze. close.

The component manufacturer conducted a 20-cycle test of -55 to 150 degrees according to the GJB128A case test, and carried out a second screening before assembly and welding, and the screening conditions were -55 ℃ to 125 ℃ for 5 cycles.

 

2 Cracking situation

After the components were assembled on the PCB, after cleaning, three preventions, power-on tests, and ESS, the diode body cracked at different positions: the body cracked axially, and its internal PN junction was intact.

 

3 Fault location

The cracked component is subjected to metallographic analysis. Through the analysis of cracking surface profile texture, the component has been subjected to mechanical or thermal stress. The possible factors for these stresses are:

a) The temperature change causes internal stress between the diode glass body and the electrode. The two kinds of stresses are too large to cause the diode glass body to rupture in the direction parallel to the diode axis due to overstress;

b) During the diode assembly and welding process, due to the pin forming and shearing, the component pins and body are forced to produce micro-cracks. After the ESS, the cracks are further deteriorated and cracked;

c) The improper installation of the diode causes micro-cracks on the component pins and body due to stress, and the cracks are further deteriorated and cracked after passing through the ESS;

d) In the temperature cycle test, the resistance body beside the diode expands and shrinks due to thermal expansion and contraction, which causes stress on the glass body of the diode parallel to the PCB surface and perpendicular to the axis of the diode;

e) The magnitude of the vibration is too large, the component and the PCB resonate to produce a tensile force perpendicular to the PCB direction, and the adhesion between the bottom of the body of the diode and the PCB on both sides of the axis is uneven, causing one side to still stick to the PCB, and the other Sideways

f) The diode is affected by some kind of external force, which is greater than the internal bonding force of the body, which causes the body to crack.

 

4 Internal stress of components

Regarding whether there is internal stress between the diode glass body and the electrode and further cracking due to the influence of temperature, it is actually a quality problem of the component itself.

 

4.1 Perform temperature cycle test on the cracked same batch of diodes without installation: high temperature ≥ 175 ℃, low temperature ≤ -60 ℃, cycle 10 times, and the test is qualified. It shows that the quality of the device itself is qualified.

 

4.2 Analysis and test verification of diode manufacturers

The manufacturer has done the following tests on cracked diodes:

a) Alternating hot flash test

In order to assess the tightness of the tube body during the rupture, an alternating hot flash test was performed on 13 cracked diodes. The test was carried out for 10 consecutive cycles, one cycle for 24 hours, and the test was qualified.

b) Thermal shock (liquid-liquid) test

In order to assess the resistance of the tube body cracked device to thermal stress, thermal shock tests were performed on 13 cracked diodes, and the tests were qualified.

c) Temperature cycle test

A sample of 1,000 similar products was taken and a temperature cycle test was performed. The high temperature ≥175 ℃, the low temperature ≤-60 ℃, the cycle 10 times, the test was qualified.

The manufacturer's conclusion: the quality of the device itself is qualified, and even the device with a damaged shell can withstand a certain environmental stress, and its functional characteristics have no signs of damage.

Whether it is the internal verification or the manufacturer verification, it shows that the quality of the device itself is qualified.

 

5 Improper shaping and cutting of component leads

The pliers are not sharp when cutting the component pins; when the component pins are formed, the forming special tools make the component pins have obvious nicks, or the deformation exceeds 10% of the pin diameter, or the root of the body is not fixed. When the lead is torsion during forming and shearing, the impact force will be transmitted to the inside of the component and cause micro-cracks in the component. During the environmental test, the applied stress will cause the micro-cracks to propagate and further deteriorate, and finally cause the body to crack.

Check the forming and shearing tooling used in the assembly and welding process, and use this tooling to shape the diode, and check if there is no crack or deformation on the component pin after cutting. After mounting and soldering this component on the PCB, perform ESS to check that there is no cracking, indicating that the component is formed and cut normally during the mounting and welding process.

 

6 Improper installation of components

There are several ways to install components: cold plate installation, printed circuit board installation, 0.08mm elevation installation; separate installation; installation with resistors close to both sides.

Through experiments and preliminary analysis, it is found that diodes mounted on the cold plate without resistance extrusion also have diode cracks, indicating that the diode cracks may also be related to the cold plate, or there may be other common reasons.

For further verification, the diode is assembled according to the following process (the installation process can make enough gaps around the diode, and the diode is completely free from squeezing):

a) There is no cold plate at the bottom of the cracked diode. If it is between the two resistors, mount the diode on the board, and install the adjacent resistors 1mm higher.

b) The cleavage diode is independently located on the cold plate and installed at a height of 0.3 mm.

c) If the cracked diode is located on the cold plate and between the two devices, the diode should be installed at a height of 2mm and the bottom should be glued.

After the ESS test, no cracking of the diode was found in the decomposition inspection.

Through further experiments, it can be known that the damage of the device casing is mainly related to the stress caused by the surrounding medium in the environmental change. By enlarging the diode installation gap (with the adjacent resistance and with the cold plate), the problem of diode cracking should be avoided. This external force may come from peripheral devices or cold plates or cured three-proof materials, etc., or may be related to vibration factors under certain conditions, so continue the test.

7 The surrounding devices expand and contract with heat and squeeze the diode

Since the general rule of PCB design is that there should be a 0.2 mm gap between adjacent components, there is indeed a case where the gap between the components is too tight and only 0.08 mm and the components are mounted on the heat sink in this cracking situation. Therefore, the thermal expansion coefficient of the surrounding devices assembled on the PCB is calculated and tested to determine whether there is a stress that squeezes the diode.

Theoretical calculation: The devices around the diode are generally metal film resistors and precision resistors. The inside of the metal film resistor is alumina ceramic, and the metal film is coated on the outside, and the coefficient of thermal expansion is negligible. The outer layer of the precision resistor is made of epoxy film plastic, with a thermal expansion coefficient of 2*10-5, and an expansion of about 0.003 mm at a temperature rise of 50 degrees. The outer layer of the diode is glass glaze, with a thermal expansion coefficient of 10-7, and an expansion of about 0.00003 mm at a temperature rise of 50 degrees. In the PCB design of this case, the distance between the diode and the surrounding devices is 0.08 mm, and through the above calculation, the component has not reached its own existing distance of 0.08 mm after expansion.

Actual test:

①Scheme: Divide the diodes into 3 groups, of which 2 groups are installed in the middle of two resistors, one group of diodes are separated from the adjacent resistor by 0.2mm, and one group of diodes are separated from the adjacent resistor by 0.08mm; the third group of single diode is attached The heat sink is installed. Perform cleaning and three preventions according to conventional techniques.

②Test: According to ESS test requirements.

③Result: There is no rupture of the diodes of the first group, and the diodes of the second and third groups have ruptures (a large number).

④Analysis: Diode cracking can be avoided by enlarging the diode installation gap, but the cracking of the third group of single installation diodes shows that there are other stresses such as the stress formed by the surrounding medium in the environment change. This external force may come from peripheral devices or heat sinks. Or the cured three-proof material, etc., may also be related to the vibration factor under certain conditions, so continue to test.

 

8 Determine whether the diode cracking is related to vibration

When the vibration level applied by the ESS test is 6.06 g, the root mean square acceleration measured in the test conducted to the product module is up to 25 g-30 g, which exceeds the acceptance vibration test value of the semiconductor device in GJB128A by 20 g. Preliminary analysis shows that the local value of the vibration test may also cause the diode to be overstressed and further stimulated by the temperature cycle, causing the diode to crack.

Actual test:

① Test: Select 5 products for the vibration test in the ESS test, and select another 5 for the 20 temperature cycle tests in the ESS test.

②Result: None of the diodes in the first 5 products are cracked. Diode cracking occurred in the last five products.

③Analysis: Diode cracking is definitely not caused by vibration. Temperature stress is a factor in diode cracking.

Diode cracking has nothing to do with whether the product is energized or not. Diode cracking will also occur if the product is not energized.

 

9 Determine whether the external force on the diode is three-proof paint or electrical stress

Select 10 products to continue the 20 temperature cycle tests in the ESS test, of which 5 are coated with three-proofing, 5 are not coated with three-proofing, and are only energized in the test.

The test results are: the first 5 diodes have cracking phenomenon, and the last 5 diodes have no cracking phenomenon.

Table 1 summarizes the test results of the previous chapters. The reasons for the diode cracking can be derived from the items listed in Table 1: The devices are densely arranged, and the three defenses cannot be uniformly applied to the diode and the gap between the adjacent devices. There are more three-proof paints in the gap between the diode and the adjacent devices. During the temperature cycle, especially at low temperature, the stress gradient formed with the surface of the device is relatively large. The hardening and shrinkage of the three-proof paint will cause lateral tension on the diode, resulting in the diode. Cracked. By enlarging the diode mounting gap (to the adjacent resistor and the heat sink) or reducing the diameter of the device body, the problem of diode cracking should be avoided.

 

11 Conclusion

The cracking of glass encapsulated components is related to the installation gap, the stress of coating the three-proof paint, and the temperature shock. According to the requirements of GJB3243, the component gap should meet the requirement of 0.8mm, so this principle should be strictly followed in the design. According to QJ165A, when components are coated, the effect of the curing of the coating material and the stress generated during temperature cycling on the coated devices should be considered, especially glass-encapsulated devices should be pre-treated before coating, so such components should be coated first The silicone with small stress effect is then coated with tri-proof paint, or when manufacturers are required to manufacture glass encapsulated devices, the glass glaze is changed from a baking enamel process to a clear paint, which is equivalent to pretreatment, which can resolve the stress of the three-proof paint on the diode body .