1. Cleaning agent and its cleaning effect detection
The main function of cleaning is to remove residual flux and residues on circuit components to prevent corrosion of the circuit. For SMA products, because of its high assembly density, small gaps between components and PCB, components and components, cleaning is more difficult. After SMA is soldered, there will always be more or less flux residues or other types of contaminants on its surface, such as plugging glue, residual glue from high-temperature tape, handwriting and dust, etc., even when using low solid content No-clean flux, there will still be a certain degree of residue. Therefore, cleaning plays an important role in ensuring the reliability of electronic products.
In order to enhance the cleaning effect, it is first necessary to select a reasonable cleaning agent and cleaning process. At the same time, it is also necessary to detect the incoming cleaning agent. The incoming cleaning agent inspection is carried out through methods such as appearance inspection and cleaning effect test.
(1) Cleaning agent detection
.Appearance inspection mainly inspects the name of the cleaning agent manufacturer, product name, standard, model, production date and shelf life, good packaging condition, suitability of the cleaning object, and whether there is deterioration due to factors such as heat or shelf life. .
In addition, the composition of the solvent will change during the cleaning process, and it will even become flammable or corrosive. At the same time, it will reduce the cleaning efficiency, so it needs to be tested regularly. The cleaning agent detection generally uses gas chromatography (GC ) Method to proceed.
(2) Measurement of cleaning rate
The following simple methods can be used to roughly determine the cleaning rate: ①Prepare a 50mm X 50mm x 1mm copper sheet test piece and weigh it, apply or soak the flux; ②After the flux evaporates, heat it in a 250C high-temperature furnace for 5min~10min, and cool it down. Weigh after room temperature; ③ Clean with the selected cleaning agent according to the relevant process specifications, and weigh after the cleaning agent evaporates; ④ Calculate the cleaning rate according to the following formula.
Cleaning rate (%)=Xiao 9^ X 10°%
Where: Wo is the weight of the copper sheet; the weight of the Wi copper sheet coated with flux and heated and cooled to room temperature; the weight of the copper sheet coated with flux after cleaning.
(3) Determination of resistivity of solvent extract
Solvent extraction test, also called ion contamination test, is the most commonly used cleaning effect test method. This test method is specified in the American standard MIL-P-28890. The principle of this test method is to immerse the test piece in an extraction solution composed of 75% isopropanol and 25% deionized water (its resistivity should not be less than 6X106n-cm), and then measure the resistivity of the extraction solution. When the resistivity It is clean when it is larger than 2xi06fi-cm.
(4) Measurement of surface insulation resistance
The surface insulation resistance is an important indicator for evaluating circuit assemblies. It checks the cleaning effect by detecting the surface insulation resistance between the components and the PCB. The test procedure of this test method is specified in the standard GB-9491. Generally, the test patterns on the standard test board are designed into a “Y” shape and a “comb” shape, as shown in Figure 2.25 and Figure 2.26. The “Y” type diagram is suitable for chip components, the test lead width is 9mil~10miKlmil = 2.54 X 10^m), and the lead spacing is greater than 10mil; the “comb” type diagram is suitable for PLCC, SOIC and other types of devices, the lead width is 9mil~10mil, Lead spacing 10mil. After the assembly and cleaning process of the standard test board is completed, it is placed under the specified temperature, humidity and voltage, and tested with a high resistance meter according to the specified procedure. The average value of three samples is usually taken as the test result.