Comparative study of methods of washing and efficient removal of flux residues. Part 1

In the electronics industry to remove water-soluble flux residues widely used processes based on pure deionized water. However, the latest industry developments and customer experience have shown that these aqueous processes no longer provide the required levels of cleanliness and do not guarantee the long-term reliability of the assembly.

INTRODUCTION

Systems wash with deionized (DI) water is becoming increasingly difficult to meet the requirements of cleanliness associated with increasing packing density of modern printed designs, the miniaturization of components and the increasing use of low-profile components. In addition, significantly increased the use of lead-free solders require higher temperature reflow. This increases the amount of residual burnt flux and tin oxide are more difficult to remove, since they do not form soluble contamination [1]. By itself, the deionized water is unable to dissolve these inorganic residues on the surface of the board and a low-profile components [2-3].

On cleaning performance affect four main factors: thermal, mechanical, detergent, washing or exposure time. For any particular cleaning process, these factors are optimized to achieve the desired result. When washing water-soluble flux residues being DI water, possible optimization options are only mechanical and thermal. If you select washing with chemicals, in addition to optimization of mechanical and thermal parameters apply different cleaners, including varying the concentration and alkalinity. Construction equipment cleaning improved, especially injectors and racks. It may also be useful to increase the temperature of wash. However, increasing temperature to 70 ° C and higher load equipment, and also significantly increases the cost of electricity.

To dissolve and remove flux residues requires that the cleaner came into contact with him. When cleaning printed circuit boards with complex geometries and dense packing deionized water with high surface tension are simply unable to get to the dirt regardless of equipment design and operating parameters of washing. At the same time cleaning additives specifically designed for use in equipment with spray in the air can penetrate into the small gaps and capillary space, better solutions, removing residues and eliminating the risk of electrochemical migration and leakage currents [2].

This paper compares the performance of low-profile components washing DI water and cleaning agent. Cleanser selected for this study, and hereinafter referred to as the means A is alkaline; It is specifically designed to dissolve flux residues at very low concentrations and temperatures washing. Assessment of purity were performed using visual analysis by measuring ionic contamination and ion chromatography. In the experiment, it was considered extremely low profile components, in which the gap between the housing and the board was less than 1 mil (0.001 inch or 25.4 mm), because they are difficult to wash. It should be noted that the visual analysis was carried out after removing all the components from the board to be able to analyze its surface under the components for residues.

Since even the board, have been tested ionic contamination and ion chromatography analysis, the operation may fail, we looked at two examples of these cases.

Hypotheses

Hypothesis 1: The water-soluble flux residues is difficult to completely remove from the low profile components (with clearance of less than 25 microns) deionized water alone.

Hypothesis 2: the increase in temperature of low profile components improves the washing results for deionized water, and for the chemical washing solution.

Hypothesis 3: A tool with a small concentration of 3-5% can improve the cleaning and increase the working range of the process.

METHODS

The experimental scheme includes an analysis of five indicators selected authors of the study in order to adequately and comprehensively assess the differences, advantages and disadvantages of solutions means A low concentration when compared to the DI water. It was chosen pipeline cleaning process. All boards were installed and melted with the five most common lead and lead free solder pastes.

PERFORMANCE INDICATORS

1. The effectiveness of cleaning

To assess and compare the effectiveness of cleaning agent A at a concentration of 3 and 5% with DI water. To test was chosen as the new test fee ZESTRON (see. Fig. 1) on which a lot of hard laundered set of components with small clearance. Reflowed boards on the conveyor and washed at different temperatures. Efficacy was determined by washing in two phases:

— inspection of the surface of the board, including under all components;

— measuring ionic contamination and ion chromatographic analysis for the parameters of washing, the best record in step 1.

2. Material compatibility

Various sensitive metal is held for 15 minutes and 24 hours in a vessel with solution A and with DI water. Material compatibility is determined visually.

3. Energy consumption

Measured load (in kilowatt hours) needed to raise the temperature from room in laundering bath to a predetermined, and for maintaining a predetermined temperature washing. Tests were carried out for four values ​​of the temperature. Energy consumption data were obtained by an industrial gauge.

4. The use of deionized water

Data on the number of DI water used in the processes of washing in pure water and chemical means have been provided by several independent sources. The analysis was based on the use of closed-DI-water equipment with hermetically sealed tanks washout. Based on these data, we compared the annual cost.

5. Impact on the environment

There are many indicators of environmental impact. One component is the amount of volatile organic compounds. Its value for money but is determined by an independent laboratory and compared to industry standards for specialized washout agents.

In addition to the mentioned analysis used information from two companies rated washout processes with chemicals. In each case, a water-soluble flux is applied by washing with DI water, and then still did not provide the required reliability of products. Moreover, it was found that the problems were caused by the reliability of flux residues on printed circuit boards. Therefore, in the experiment were assessed and compared the results of washing boards DI water and wash with cleaning agent A.

Basic research Performance indicator 1: effectiveness of washing

The purpose of the first study was to determine the efficiency of washing with deionized water and compared with the washing agent A washing Efficacy was evaluated in two stages:

— test boards were prepared and washed for visual analysis of the surface and area under all of the components;

— Test boards were prepared, washed and evaluated by means of ion contamination and ion-chromatographic analysis. For the first stage 60 test boards ZESTRON (see. Fig. 1) have been established (four each) chip capacitors 0402, 0603, 0805, 1206, 1210, 1812, 1825, 6032 and transistor SOT-23 package, ie . A total of 36 components to the board. For the second phase of the study were 40 boards ZESTRON completely filled all the components. All components are classified as low-profile with a clearance of less than 1 mil (25.4 microns).

The new test fee ZESTRON has been specially designed for testing of complex situations washing.

For this study, 10 have been selected most frequently used lead and lead-free solder paste on the basis of comparative studies of the preceding wash with DI water and a low concentration of reagents. Lead pastes were designated by letters A, B, C, D and E, lead-free — the letters F, G, H, I, J. paste reflowed in accordance with the profile of Tables 1 and 2.

Visual inspection

After reflow were tested washing stage 1 were washed 60 boards from 100: 30 of them — with deionized water and 30 — means A. The tests were conducted in a conveyor installation washing with four uprights with a V-sprinkler nozzles. Parameters of the washing process are shown in Table 3. After washing inspect the board surface and the area under all of the components in accordance with IPC-A-610, Rev E-2010 [4].

ANALYSIS: visual inspection

In step 1,

The analysis was carried out by washing one indicator: is fully removed flux residues under each component. If so, the component classified as a net and placed a table. For each of the liquid in laundering and cleaning at every temperature required in total, inspect place under 180 components for lead and lead free solder pastes. All the results are shown in Table 4. In addition, cleaning results are plotted against the washing liquid, washing temperature, pastes and types of components. Analysis by type of component detail is shown in Figure 2-7, and the analysis of the paste — in Figures 8-13. The graphs show the number of components, the space beneath which was completely cleared, it was checked when removing components from the test boards.

ANALYSIS OF washing DI water

Inspection of the surface:

— at each temperature and for all lead pastes substrate surface is clean;

— each temperature for lead-free pastes a slight white residue was observed around the various sites for three of the five types of pastes. Inspection of the area under the components after lead paste:

— completely cleaned surface under all components at all temperature values ​​using the paste D (see. Fig. 8-10).

— washing quality actually decreases with increasing temperature, as evidenced by washing off the components 1825, 1812,1210,1206 and 0805 (see. Fig. 2-4). The results of the inspection area under the use of lead-free components after the paste:

— completely cleaned surface at all components in all temperature scenarios using pastes F and G (see. fig. 11-13).

— an increase in temperature does not improve the quality of the cleaning of larger components and in fact affects the results for sizes 6032 and 1812 (see. Fig. 5-7).

— an increase in temperature does not improve the washing results. In fact, at a temperature of 66 ° C laundered minimal quality of the lead and lead-free components, than at 50 ° C, which reduces the percentage of washed components to 68% at 50 ° C to 63% at 66 ° C for lead-acid components and 85% 50 ° C to 71% at 66 ° C for infinite-lead components (see. Table. 4).

ANALYSIS OF washing solution A (3%)

Inspection of the surface:

— for each value of temperature and all kinds of pastes surface was completely washed off, white residue was observed.

Inspection of the area under the components after lead paste:

— completely cleaned surface under all components in all temperature scenarios using toothpastes D and E (see. Fig. 8-10);

— completely cleaned surface under all components at 66 ° C and using pastes A, C, D and E (see. Fig. 10);

— temperature increase improved washing results both at 57 ° C and at 66 ° C (see. Fig. 9-10). Inspection of the area under the components after lead-free paste:

— completely cleaned surface under all components in all temperature scenarios using pastes F, G, I and J (see. Fig. 11-13);

— completely cleaned surface under all components at 66 ° C and using pastes F, G, H, I and J (see. Figs. 11-13).

The effectiveness of cleaning a component for improving the most complex components with increasing temperature. Indeed, the area under all of the components were completely clean at 66 ° C for all lead free solder pastes (see. Fig. 7).

ANALYSIS OF washing solution A (5%)

Inspection of the surface:

— for each value of temperature and all kinds of pastes all sites on the surface were completely washed, white residue was observed.

Inspection of the area under the components after lead paste:

— completely cleaned surface under all components at all temperature values ​​using pastes D and E. When using the paste A site for all of the components completely washed off at 57 and 66 ° C (see. Fig. 9 and 10).

— completely cleaned surface under all components at 66 ° C and using pastes A, C, D and E (see. Fig. 10).

Inspection of the area under the components after lead-free paste:

— completely cleaned surface under all components for all values ​​and temperatures using pastes F, G, I and J (see. Fig. 11-13); all components fully washed at 57 and 66 ° C (see. Fig. 9 and 10).

— completely cleaned surface under all components at 66 ° C and using pastes A, C, D and E (see. Fig. 10).

Inspection of the area under the components after lead-free paste:

— completely cleaned surface under all components for all values ​​and temperatures using pastes F, G, I and J (see. Fig. 11-13);

— completely cleaned surface under all components at 57 ° C and bb and pastes using F, G,

H, I and J (see. Fig. 12-13);

— washing efficiency the most complex components is achieved by increasing the temperature (see. Fig. 5-7).

The performance as a cleaning agent A at a concentration of 3% and 5 support the hypothesis authoring 2 that raising the temperature can improve the washing results thereof. In addition, it confirms the fact that the detergent is designed to dissolve water-soluble flux residues, combined with low surface tension allows the reagent to penetrate the gap and capillary space, thus freeing and removing stuck residues.

Figures 14-18 are photographs of the results under the washing components with DI water, and means for A paste B (lead) and J (Pb-free). The photos show the classification results from fully washed to the available balance. These photos of the author chose both successful and unsuccessful results of cleaning under large components 6032, 1825 and SOT23.

End of the article in the next issue of «production of electronics.»

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