Electronics Assembly Knowledge, Vision & Wisdom
Determining the Effect of Fine Mesh Solder Powder on Flux Residue Removal
Determining the Effect of Fine Mesh Solder Powder on Flux Residue Removal
There is a drive to miniaturize solder paste deposits. A key element to success is the incorporation of smaller solder particles in the solder paste.
Materials Tech

Materials Tech programs cover topics including:
Adhesives, Chemicals, Cleaning Solutions, Coatings, Components, Design, Embedded Technology, Fasteners, Finishes, Flex Circuits, Flip Chip, Fluxes, PC Fab, Solders, Solder Masks, Solder Paste and more.
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Authored By:
Timothy O’Neill
AIM
Cranston, RI, USA

Terry Munson
Foresite
Kokomo, IN, USA

Kalyan Nukala, M.S.Ch.E.
ZESTRON Americas
Manassas, VA, USA

Ravi Parthasarathy, M.S.Ch.E.
ZESTRON Americas
Manassas, VA, USA

Summary
Solder paste users and developers face an unrelenting drive to miniaturize solder paste deposits. A key element to success is the incorporation of smaller solder particles in the solder paste. New flux formulas have been developed in parallel to enhance printing and soldering performance with finer powder. The focus of this development has been primarily on ‘No-Clean’ chemistries as this flux technology has emerged as the industry standard. While No-Clean flux residue is designed to be left in place after soldering, there are a variety of end use applications that require its removal including coating applications, RF and high voltage circuitry. When flux residue removal is mandated, wash chemistry is required in order to solubilize flux residue so that it can be washed and rinsed away.

At APEX 2018, a study titled ‘Jet Printed Solder Paste and Cleaning Challenges’ was presented whereby data was presented indicating that as solder powder becomes finer, the resulting flux residues become more difficult to remove. As a continuation of the APEX 2018 study, this study will test a common No-Clean flux chemistry with progressively finer SAC305 solder powders with a variety of cleaning chemistries and methods to attempt to quantify the implications of finer mesh powder on flux removal. This study was divided into two phases. For Phase 1, a fully populated ZESTRON test vehicle was assembled and cleaned. Post washed cleanliness levels on the surface as well as under-component were measured using visual inspection.

Based on this analysis, Phase 2 trials were conducted utilizing the IPC-B-52 test vehicle cleaning agents yielding best and worst cleaning results from Phase 1. In this phase, cleanliness assessment was conducted using SIR and Ion Chromatography analyses. The results were analyzed to assess the influence of finer solder powder paste deposits on the cleaning process and materials. All cleanliness assessments were conducted in accordance with current IPC guidelines.

Conclusions
Effective removal or cleaning post solder flux residues is certainly critical for reliable performance of Class III electronic assemblies. It is well known that residues left behind can have a negative impact on reliability caused by either electrochemical migration or by corrosion failure mechanisms [2]. Thus, ensuring that post solder flux residues are completely removed from the substrate is critically important to the long-term reliability of the electronic assemblies.

The main goal of this study was to assess the effect of fine mesh powder on flux residue removal. In this case, two versions of a single No-Clean solder formulation were used, one formulated with Type 4 and another with Type 6 solder powders. However, each was formulated for screen printing. The cleaning process selected was a spray-in-air system that compared six different cleaning agents.

Through the Phase 1 trials and using populated test vehicles, under-component visual inspection was used to identify the best and worst combinations of cleaning agent type and cleaning process operating parameters. It is important to note that the authors did not optimize the cleaning process in order to achieve best results. Rather, industry standard operating parameters were selected and maintained constant throughout all trials as the intent of the trials was a comparative analysis between the two types of fine mesh solder powders. Based on visual analysis, Cleaning Agent A produced the best results for all Phase 1 cleaning trials. Reference Table 11.

Within Phase 2 of the study, IPC-B-52 coupons populated with SMT components were used and cleaned with the down selected cleaning agents from Phase 1. Both SIR and IC analyses were performed on all test coupons. For this analysis, baseline coupons (uncleaned) for both the Type 4 and Type 6 solder powder were also produced and analyzed.

It was interesting to note that all the Phase 2 test coupons had passing values for SIR and IC. As evidenced by the passing results of the SIR data for the Baseline coupons (Figures 21 and 22), the reflow process produced effective soldering results for both the Type 4 and Type 6 solder pastes.

With regard to SIR, the values remained steady or increased over time in all cases other than baseline (uncleaned) regardless of the cleaning agent used. Coupled with the passing IC results, the authors deduced that all cleaning agents were effectively rinsed thereby leaving no or minimal trace of ionics on the substrate surface. However, under-component visual inspection of the baseline components compared to the coupons that were cleaned revealed flux residues (Figures 33 - 36). Even though SIR and IC analysis yielded passing results, post reflow flux residues remained under-components in all but the best case scenarios utilizing Cleaning Agent A (Trials 5 and 6).

Focusing on Cleaning Agent A, the authors selected three component groups from the SIR coupon for further analysis (Figure 23). The SIR data for the best case scenario (Trials 5 and 6) for each solder paste type within the three component groups is detailed in Figures 30 - 32. In the case of the C1 - C8 and U3 component groups, the SIR values are the same for each paste type. With regard to the C39 – C53 component group, higher SIR values were realized for the Type 4 solder powder. Thus, each solder paste type was effectively cleaned.

In addition to this, under-component visual inspection was conducted on several of the C54 – C63 components examining the worst case cleaning scenarios, Trials 13 and 16 where Cleaning Agent C was used. Interestingly enough, even though IC analysis yielded passing results, flux residues were visible under components for both solder paste types.

This study confirmed that post reflow flux residues can be effectively removed as demonstrated through the use of the IPC-B-52 test coupon and resulting SIR and IC analysis.

However, cleaning agent selection and an effective cleaning process are critical to achieving the desired results. Although six different cleaning agents were used in this study, all yielded passing results for both SIR and IC analysis. However, under-component visual inspection analysis varied. Cleaning Agent A produced the best overall results.

Future studies will involve halide based water soluble pastes with different powder sizes.

Initially Published in the SMTA Proceedings

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