Printed circuit board cleaning quality inspection (4)

2) Test method for insulation resistance of printed circuit boards

According to GB/T 4677.1 implementation.

A, sample pretreatment

- Samples are placed under normal atmospheric conditions (as specified in GB/T 2421) for more than 24 hours;

- Constant moist heat conditions: According to the provisions of GB/T 2423.3 carried out. If not in the box, it should be restored under normal test atmospheric conditions for 2 hours.

B, the atmospheric conditions of the test:

a) Normal test atmospheric conditions: According to the provisions in GB 2421:

b) Propelled test atmospheric conditions: temperature 230C: relative humidity 48%-52%;

Pressure 86kPa-106kPa (According to the provisions of GB/T 2423. 3;

c) constant moist heat conditions: according to the provisions of GB/T 2423. 3;

d) Dry heat conditions: According to relevant regulations.

C, test voltage: (10 ± 1) V, (100 ± 15) V, (500 ± 50) V.

D. Test procedure: Calibrate the instrument and add the test voltage to the sample for 1 minute before measurement. If a stable reading can be obtained early, measure it early. If it does not get a stable reading, it should be in the report. Record this phenomenon.

3) Test methods for surface insulation resistance of polymer solder masks and conformal coatings

It can be implemented according to IPC-TM-650-

Test board: IPC-B-25A test board;

Environmental conditions: T, H grade (200C-270C) and 40%-50% RH;

Test Conditions:

T level: (65 ± 2) 0C, (90 ± 3) 0C, no bias, 24 hours;

Grade H: 250C-(65±2) 0C, 90% RH, 50VD. C. bias, thermal cycle, 6 days and 16 hours;

Testing process:

A, the sample was placed in an oven (50 ± 2) 0C, without humidification, the sample was allowed to cool under atmospheric conditions after 24 hours;

B, add 100VD. The C voltage measures the insulation resistance between points 1-2, 2-3, 3-4, and 4-5 of IPC-B-25A and two comb test boards with only two test points and "Y" test. Insulation resistance of the board (reading after 1 minute);

C, temperature and humidity experiments:

Grade H: Put the test board vertically into the oven. At least one bare board with no coating is used as a reference. Connect IPC-B-25A 1, 3, and 5 to the positive pole. Connect the 2 and 4 connections. The negative electrode is biased; two comb test with only two test points and two test points of the “Y” test board are respectively connected with the positive and negative voltages. After closing the oven door, increase the bias to 50VD. C thermal cycle 20 times. The humidity is kept at least 80% during the cooling phase (iii) and at least 85% during the rest of the thermal cycle. Thermal cycling conditions are as follows:

A, starting from 250C, rose to 650C in 1 hour and 45 minutes;

B, Incubate (3-3.5) hours at 650C;

C. Cool down from 650C to 250C in 1.75 hours.

Note: The thermal cycle continues without stopping in the middle.

Grade T: The test panels were placed vertically in an oven with at least one uncoated bare panel as a reference and held at 90% RH for 24 hours at 650C.

D, measurement:

Class H: Measured every 24 hours during the high temperature holding phase of the thermal cycle. The polarity of the measured voltage is the same as the bias voltage, and the oven door cannot be opened. After the temperature and humidity test was completed, the bias was removed and the sample was taken out of the oven. After recovery for 1 hour under atmospheric conditions, B was pressed before 2 hours. The requirements are measured.

T level: According to the requirements of B, add 100VD. C. Voltage measurement (take a reading after 1 minute).

E, sample evaluation:

Level H: Based on the final test data in the oven and the test data after recovery under atmospheric conditions.

Level T: Based on the average of the test data.

4) Visual inspection

After the surface insulation resistance test is completed, remove all components that have been biased (Note: Chemical and heating methods must not be used). The printed board was subjected to full-area etching and dendrite inspection using a 10X-30X magnifier. There should be no obvious corrosion on the printed board. If dendrites form, their size should not exceed 20% of the pad or wiring pitch. The surface coating should not be deteriorated, cracked, and stained.

Selection of test methods

The extraction solution resistivity test method was established by Hobson and DeNoon in the early 1970s as a tool for process control and was the earliest method of surface ion contamination testing.

However, in 1998 Dr. Per-Erik Tegehall of Sweden conducted the following experiments: Three PCBs were selected from each PCB manufacturer, of which two were tested according to the standard method and the other one was subjected to ultrasound during the extraction process. The surface ion pollution test results are shown in Table 6.

Table 6 MgNaCl/cm2

Test results of printed board manufacturers' standard methods Apply ultrasonic test results
A 0.055,0.066 1.88
B 0.269, 0.331 2.73
C 0.922, 3.12 3.12
D 0.103, 0.112 1.35

Re-select sample A, ultrasonic extraction for 2 hours, the surface ion contamination of 5.06μgNaCl/cm2, change the test solution, and then ultrasonic extraction for 2 hours, the surface ion contamination of 1.59μgNaCl/cm2, the total surface ion pollution is 6.65μgNaCl/cm2 . This experiment shows that only a part of the contaminants remaining on the printed board is extracted during the test. This test method has a large error and is not suitable as a criterion for the qualification of the product.

According to the US military standard MIL-STD-2000A, the extraction solution resistivity test method was developed for punched mounting printed boards using rosin-based flux welding and CFC cleaning agents. This method and its quality criteria do not apply to surface mount and mixed printed boards. The main reason is that the result of the extraction solution resistivity method is the average of surface ion contamination. The surface mount printed circuit board solder flux is only applied to the pad, and the lower part of the surface mount device is difficult to clean, so the flux residue distribution is extremely uneven, the results of the extraction solution resistivity method can not be explained Surface mount device around. At the same time, it is also pointed out that the extraction solution resistivity method and its quality criteria are not applicable to printed boards that are soldered with non-rosin-based fluxes. However, with the completion of further reliability studies, this view has been negated by other standards including J-STD-001B.

Although the error of extract solution resistivity (ROSE) test method is relatively large, it has the advantages of low cost, short test cycle, non-destructive testing of samples, and having a large amount of historical data. It is the best method for on-line detection in the production process.

Ion chromatography, infrared spectrophotometry, UV spectrophotometry, and high performance liquid chromatography can qualitatively and quantitatively detect various ions, organic substances, and their contents in contaminants. For unqualified products, the source of major pollutants can be quickly and effectively determined based on these data, which is an important method for process analysis.

The surface insulation resistance test method accelerates the simulation of the working environment of the printed board, and can obtain information on the electrical properties of the printed board, surface corrosion, surface coating, etc., and is more scientific as a quantitative indicator of the reliability of the printed board.

Source: STM Forum

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