Main test items and test methods of incoming adhesive for smt processing
1. Test items for adhesives
The role of the adhesive is to ensure that the components can be firmly adhered to the PCB. Starting from the process requirements of SMT, it should have suitable viscosity, low slump, fast curing, moderate bonding strength, high temperature resistance, and good Stable electrical and chemical properties, no peculiar smell, distinguishable color, stable storage and other properties.
For this reason, the main items of adhesive testing include viscosity and thixotropy coefficient, bonding strength, spreading and slumping, curing time, electrical properties, and appearance testing for deterioration.
Adhesive
2. Test methods for main test items of adhesives
(1) Viscosity and thixotropic coefficient
The viscosity and thixotropy coefficient of the adhesive are related to the bonding strength, coating performance and spreading performance. They are important parameters of the adhesive. The viscosity and thixotropy coefficient of the adhesive can be measured with a rotary viscometer according to relevant standards. The detection method is the same as that of the viscosity and thixotropic coefficient of solder paste. The viscosity of the adhesive for SMA is generally around 400 Pa-s.
(2) Bonding strength
Bonding strength refers to the strength required to bond the components to the PCB to ensure that they will not fall off due to vibration and thermal shock during the process. For chip resistors or capacitors, the bonding strength is generally required to withstand a tensile force of 8N-20N. It can not only ensure that the components will not fall during wave soldering, but also ensure that they can be easily removed during rework.
The bond strength test can be carried out through the bond strength test. The simplest test method is to use a small torsion tester or a tension tester, that is, use a small torsion tester to clamp the bonded solidified component and twist it until it falls off and record the reading, or use a tensioner to stretch the bonded solidified component until it falls off and record the reading.
The tensile shear strength of the adhesive can also be tested by the tensile shear test. The shear strength of the adhesive is generally 6MPa~10MPa. The principle of the tensile shear test of the adhesive and the size of the test specimen are shown in Figure 2.23. The parts generally use two metal sheets with exactly the same material specifications, such as LY12-CZ aluminum alloy or lCrl8Ni9Ti stainless steel, 45 carbon steel and other materials. After the glued shear surface is polished smoothly and smoothly, the glue is applied for about 0.1g. After the glued, a pressure of about 0.1MPa is applied, and it is cured for 1h under the condition of 120t ± 2C, and the tensile test is carried out after it is completely cured. Accurately clamp the tensile clamping surface in the symmetrical holder of the tensile shear testing machine, load it at a stable speed of 5mm/min ± Imm/min, record the specimen to reach the maximum load of shear failure and press the following formula Calculate the tensile shear strength.
(3) Spreading and collapse
The standard “SJZH1187-1998 General Specification for Adhesives for Surface Assembly” specifies the adhesive spreading and slumping test methods as follows: ① Prepare two pieces of clean ground glass with dimensions of 76mm x 25mm X 1mm, and apply more than three points on them respectively. Glue point with a diameter of 6.5mm and a thickness of 0.25mm; ②Use a sample for spreading test, and place it in an environment with a temperature of 25C±5C and a relative humidity of 50% ±25% for 50min~70min to test the increase in the diameter of the glue point Percentage, this value is the spreadability of the adhesive; ③Use another sample to carry out a slumping test and cure it according to the conditions specified by the adhesive manufacturer, measure the diameter of the glue point after curing, and increase the percentage of the glue point diameter The number is the slump of the adhesive.
(4) Curing time
The curing time of the bonding agent refers to the time required for the bonding agent to achieve the expected bonding strength and other properties through a chemical reaction. It can be determined by the following simple method: take a sample of about 0.5g and spread it on a 50mmX25mm aluminum foil with an area of 35mm x 8mm. , Put it into an electric heating blast drying oven with a constant temperature of 150C, and use a thin glass rod to probe until curing and record the time.
(5) Electrical performance
The adhesive directly contacts the components and PCB, or it may directly contact the component pins and PCB pads. Its electrical performance will affect the performance of SMA products, so strict requirements are imposed on it. The content of the electrical performance test of the adhesive includes withstand voltage, dielectric constant, dielectric loss factor, volume resistivity, surface resistance, moisture and heat resistance, insulation resistance, and electromigration. These tests can reflect the electrical properties of the adhesive from different angles. performance.
The test of the withstand voltage and electrical strength can be carried out according to the method specified in the SJ/T 10309 standard; the test of the dielectric constant and the dielectric loss factor can be carried out according to the method specified in the GB1409 standard; the volume resistivity and surface resistance can be carried out according to the method specified in the GB1410 standard Proceed; the thickness of the test adhesive layer is generally 0.38mm ± 0.05mm, and the test frequency is 1kHz and IMHz. Among them, the volume resistivity is a commonly used test item for evaluating insulating materials. It is the quotient of the DC electric field strength in the insulating material and the steady-state current density, reflecting the volume resistance per unit volume of the insulating material. The volume resistivity can not only check whether the insulating material is uniform, but also check the conductive impurities that affect the quality of the material.
The test of heat and humidity resistance and electromigration can be carried out according to the method specified in the SJ/T 11187 standard. Figure 2.24 is the test pattern for humidity and heat resistance insulation resistance test. The basic test principle and procedure are: ① Prepare 4 or more test specimens with a line width of 0.635mm and a spacing of 1.27mm (if necessary, the line width and spacing are both 0.5mm) ), which contains a non-coated adhesive for comparison; ② Apply adhesive on the test piece according to the process specification, with a thickness of 0.25mm ± 0.05mm, and place it at room temperature for 24 hours after curing according to the conditions specified by the manufacturer; ③The test piece is treated at a temperature of 501C ±5X3 without humidification for 24 hours and then cooled to room temperature. Connect the positive pole at the 1, 3, and 5 points of the graph, and the negative pole at the 2, 4 points, press 100V DC voltage Imin and disconnect the power supply to test. Its surface resistance; ④Keep the test piece in a test box with a temperature of 50C ±5X3 and a relative humidity of 90% for 7 days, and at the same time add a 100V DC power supply, and test the thermal insulation resistance every 24h.
When SMT is tested by the above-mentioned method of bonding agent, the electric strength should be no less than 10kV/mm; the dielectric constant should be no more than 5.5; the dielectric loss factor should be no more than 0.035; the volume resistivity should be greater than 1.0 X 1012Q•cm; the surface resistance should be Greater than 5.ox iolon; heat and humidity resistance insulation resistance should be greater than i.ox io8n0
(6) Appearance inspection
The appearance inspection of adhesives includes routine inspections before purchase and use, and visual inspections after the adhesive is cured. Routine inspections before use include descriptions of performance parameters such as variety, model, production date and shelf life, packaging and viscosity indicators, and whether the adhesive is uneven, foreign matter, or abnormal in color. The visual inspection after bonding and curing is mainly to observe whether the surface of the cured glue point is hardened, smooth, and level, and whether there are defects such as pinholes and bubbles.