PCB cleaning effect detection methods and evaluation criteria

Published 04/14/2014 | By chen yuxun

PCB production, processing and circuit boards design and other aspects of the system board are involved in the PCB circuit board cleaning effect detection. PCB circuit board cleaning effect on detection methods and evaluation criteria, electronic processing and circuit board engineers should follow certain standards. The following is the correct method for detecting PCB circuit board cleaning effect detection and scientific evaluation criteria:

First,raw materisl and pcb cleaning requirements

1.Raw material quality requirements

1) Tin – lead solders —– pressure processing Tin – lead solders chemical composition must meet GB / T 31311 requirements. Casting Tin – lead solders chemical composition must meet GB / T 8012 requirements.

2)Solder flux—– About the quality of the flux, it should be detected from the appearance, physical and color stability, non-volatile content, viscosity and density, the resistance value of water extraction, the halogen content, solids content, of the flux dryness, copper mirror corrosion resistance, insulation resistance, and other aspects of ionic contamination.

2 PCB cleaning quality requirements

In the current widespread use of industry standards, the quality of the cleaning printed circuit board has the following requirements.

1) J-STD-001B requirements: A, plasma levels of pollutants: <1.56μgNaCl/cm2; B, the amount of flux residue: a <200μgNaCl/cm2, two <100μgNaCl/cm2, three <40μgNa-Cl / cm2; C, average insulation resistance> 1 rating 108Ω, (log10) standard deviation <3.

2) IPC-SA-61 according to the process prescribed value.

3) MIL-STD-2000A requirements ion contaminant levels <1.56μgNaCl/cm2.

Further, in the specification MIL-P-28809, the predetermined washing or cleaning fluid can also be a solution of the resistivity as a criterion for the cleaning. Cleaning solution resistivity greater than 2 ratings 106Ω.cm is clean, otherwise it is not clean. This method is suitable for monitoring the cleaning process. Due to a variety of commercial tester appeared for testing the ionic contamination on the surface, the test results from different test systems are different. But higher than the manual test results. Therefore, proposed the concept of equivalent coefficient to achieve the test results can be compared between different systems.

4) Content Process: ion contaminant levels, the amount of flux residue

Process A <1.5μgNaCl/cm2 <217μg / board

Process C <2.8μgNaCl/cm2 <2852μg / board

Process D <9.4μgNaCl/cm2 <1481μg / board

The average value of insulation resistance

> 1 comment 108Ω, (log10) standard deviation <3

Note:

1 Process A: PCB bare board – Testing;

2 Process C: PCB bare board – SMT – Reflow – Cleaning – Testing;

3 Process D: PCB bare board – SMT – Reflow – Cleaning – soldering – Cleaning – Testing;

4 test board for the IPC-B-36.

Second, the detection method

1.Visual inspection without using a magnifying glass,observing direct printed circuit board surface with or without the presence of visible residue .
2 surface ionic contamination test method.

(1) extraction solution resistance (ROSE) test method: It is the principle to add 25% to 75% isopropanol in deionized water (volume ratio) of the test solution. Washing the printed circuit board surface contamination and the residue is dissolved in the surface of the PCB to the test solution. Since these positive and negative ions in the pollutants reduce the resistivity of the test solution, the more dissolved ions,the lower the resistivity , both having inverse function. Test liquid ion equivalent = constant / resistivity of the test liquid

(2) Using this function. It can calculate the surface of the printed circuit board of the content of residual ions, by measuring the resistance value before the test liquid and the rinsing liquid used in the test volume. And provides NaCl equivalent per square centimeter to indicate that μgNaCl/cm2. A, manual testing method according to GB / T 4677.22 perform or refer to IPC-TM-650 in 2.3.25, MIL-STD-2000A execution. Ratios per square centimeter of the printed circuit board take 1.5ml of the test solution. The resistivity of the test solution must be greater than 6MΩ.cm.

Flushing the PCB surface like thin stream until the test solution was collected from all the beaker. This process requires at least 1 minute. Electrical conductive bridge or equivalent range and precision instruments to measure the resistivity of the test solution, according to formula (5-2) NaCl equivalent calculated on the unit area. Wr = 1.56 Rating 2 / p ……….

Formula: Wr – NaCl equivalents per square centimeter on, μgNaCl/cm2;When a sample containing 1.56μgNaCl/cm2 the resistivity, MΩ.cm; P – collection fluid resistivity, MΩ.cm; a value of 1.56-resistance per unit area of the specimen contained 2MΩ.cm corresponding NaCl equivalent, μg/cm2.

B, instrument test method：according to IPC-TM-650-2.3.26 perform or refer to IPC-TM-650-2.3.26.1 execution. Determined by measuring the content of isopropanol in the test liquid temperature and density, and allowed to reach 75%. Open the purge pump, the ion exchange column purification, until the resistivity of the test solution reaches or exceeds 20MΩ.cm.

After the system verifies correct, pouring the amount of test fluid into the tank and placing the test sample.Turned on the Test Pump and measuring the resistivity until it achieved stable.

C,The data processing circuit according to the structure of the different test cycles. The test can be divided into static and dynamic testing method testing method. Cycle route back to test slot static test method, resistivity test probe and test the pump constituted. NaCl equivalent per unit area is calculated according to a formula. Formula: NaCl equivalents per square centimeter area Wr-on, μgNaCl/cm2. V-circulating loop test volume of the test solution, L; p1 – the final value of resistivity of the test solution, Ω.cm. S-test sample size (length assessment wide assessment 2), cm2. The initial resistivity value of po-test liquid, Ω.cm. C – amount of isopropanol in the test solution (75%); A, B – experimental constants.

Dynamic test method test cycle route back consist of test tank, electrical resistivity test probe, test pumps and ion exchange column. Throughout the entire testing process, the test solution through an ion exchange column kept purification. Therefore, the test process should be continuously measured resistivity of the test liquid, and incremental. The amount of ions extracted found formula.

(4) Relationship. The formula: N – ion amount in the test solution, moL; test V-Volume of test solution in the circulation loop, L;; Resistivity of P1-t k-value test experimental constants. 2) Test method according to ion chromatography IPC-TM-650 performed in 2.3.28. Experimental equipment used include: A, ion chromatography; B, a hot bath package: 800C ± 50C; C, polyethylene sealable plastic bag: extractable contaminants <25mg/kg; D, polyethylene plastic bags: Cl – <3mg/kg; E, deionized water: 18.3MΩ.cm, Cl-<50mg/kg; F, isopropanol: electronic grade. Configuration of 75% isopropanol plus 25% deionized water (volume ratio) extraction solution, the printed circuit board and (100-250) mL extract was placed in polyethylene bags (printed circuit board should be fully immersed in the extraction after the solution) and heat sealed in (80 ± 5) 0C for 1 hour in a hot water bath package. Remove the plastic bag into the extract were tested in ion chromatography, ion content according to the formula.

(5) Calculation. The formula: Wr – ion content on a per square centimeter, μgNaCl/cm2. C-content of the tested according to the standard of a liquid ion extraction, mg / kg; V0-poured into a polyethylene bag in the volume of the extraction solution, mL; V1-injected into the ion chromatograph extracts were tested volume, mL; S-printed circuit board area (length assessment wide assessment 2), cm2.