-
- News
- Books
Featured Books
- smt007 Magazine
Latest Issues
Current IssueBox Build
One trend is to add box build and final assembly to your product offering. In this issue, we explore the opportunities and risks of adding system assembly to your service portfolio.
IPC APEX EXPO 2024 Pre-show
This month’s issue devotes its pages to a comprehensive preview of the IPC APEX EXPO 2024 event. Whether your role is technical or business, if you're new-to-the-industry or seasoned veteran, you'll find value throughout this program.
Boost Your Sales
Every part of your business can be evaluated as a process, including your sales funnel. Optimizing your selling process requires a coordinated effort between marketing and sales. In this issue, industry experts in marketing and sales offer their best advice on how to boost your sales efforts.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - smt007 Magazine
STEP 7: Flux Chemistry for Lead-free SMT
December 31, 1969 |Estimated reading time: 7 minutes
By Edward Briggs and Ronald Lasky, Ph.D., Indium Corporation
SAC alloys have a higher melting point and exhibit poor wettability compared to tin/lead eutectic alloys. Flux chemistries are ever evolving to meet this changing environment. The purpose of this paper is to discuss the function of flux chemistry, acknowledging the general ingredients that make up most no-clean materials.
To meet the European Union (EU) directives WEEE and RoHS, high-tin-containing alloys have entered the market as lead-free soldering options. For SMT, the more popular of these high-tin options are the tin/silver/copper (Sn/Ag/Cu – SAC) alloys. Unfortunately, SAC alloys have a higher melting point and exhibit poor wettability compared to tin/lead (Sn/Pb) eutectic alloys. Flux chemistries are ever evolving to meet this changing environment.
The implementation of lead-free processes demands advanced chemistries capable of enduring the higher reflow temperatures of SAC solder alloys, as well as removing the higher percentage – as compared to tin/lead eutectic – of tenacious tin oxides present in SAC alloys. The chemistries also must compensate for poorer wetting and increased voiding often associated with high-tin, lead-free materials.
The purpose of this paper is to discuss the function of flux chemistry, acknowledging the general ingredients that make up most no-clean materials. Of particular interest are halide chemistries because of their superior ability in removing oxides. The difference between free halide and covalent halide is discussed briefly, as well as the proper or practical test methods for halides in no-clean chemistries.
Function of Flux Chemistry
Flux chemistries serve many complicated functions. They remove surface oxide, promote wetting, transfer thermal energy, provide an oxidation barrier during reflow, and encapsulate post reflow. Simultaneously, they act as a carrier or medium to suspend solder powder particles, provide tack to hold components, and promote workability (physical stress exerted through printing or dispensing).
General Categories of Flux Ingredients
To perform these functions, no-clean flux chemistries incorporate certain categories of chemicals: rosins/resins, rheological additives, solvents, and activator/s or activator packages.
Rosins and resins (modified or synthetic rosins) usually are medium- to high-molecular-weight compounds. Tacky in nature, they provide some fluxing activity because they are generally 80-90% abietic acid. Rosins and resins also act as oxidative barriers during reflow, protecting the newly “cleaned” surfaces created by the activators.
Rheological additives, sometimes referred to as gelling materials, affect the adhesive and cohesive properties of the solder paste. Adhesive properties are related to external attractions, e.g., the solder paste to squeegee blade or stencil. Cohesive properties are related to internal attractions, e.g., the attraction of solder paste particles to one another. In combination, these forces define the ability of the solder paste to flow when work is performed on the material, as well as the resistance to flow or slump after paste deposition. They also define the ability of paste release from the aperture as well as the tackiness that holds the components in place during reflow.
Specific rheological additives typically are selected from those materials that display thixotropic properties. Thixotropic materials have higher viscosity at rest and lower viscosity when work energy is applied to the paste by physical shearing. The viscosity of thixotropic rheological additives temporarily decreases as the material is sheared by mixing or by paste rolling as the squeegee pushes the paste across the stencil. Once the paste has been printed, the viscosity quickly returns to its original value. This property ensures sharp print definition at the lower viscosity and low cold slump and good tack when the paste returns to its higher viscosity.
Figure 1. Low boiling point solvents cause decreased stencil life and increased tack inconsistency.
Solvents act as a medium to dissolve the other categories of materials. They also aid in creating a homogenous mixture and affect such paste attributes as viscosity, tack, and stencil life. Low boiling point solvents evaporate easily into the surroundings and cause decreased stencil life and increased tack inconsistency (Figure 1).
Activator/s or activator packages prepare the surfaces to be joined by dissolving metal oxides. The activator should be inactive at room temperature. This will promote good shelf life and open stencil life. It should not volatize prior to reflow and should fully activate at reflow. Activators are generally organic acids and/or bases and organic halide salts or compounds.1
Effect of Halide on Shelf and Stencil Life
Halides are exceptional at effectively dissolving oxides; however, because of their reactivity, free, or ionic, halides can decrease paste shelf life and open time on stencil.
Free halides exist as ions due to complete transfer of electrons, forming negatively charged particles. Ions, therefore, are very reactive. Covalent halides involve the sharing of electrons. Covalent halides affect the overall charge of a molecule, but the influence is dispersed over the molecule, or a portion of it. The result is that covalent halides are generally more stable at room temperature, yet still provide exceptional oxide removal once activated immediately prior to reflow. The effect is better shelf life and open stencil time.
Long-term Free Halides and Reliability
Free halides are most associated with reliability issues caused by dendritic growth and current leakage. In the late 1980s and early 1990s, IBM and AT&T associated free halides with dendritic filament shorts.2 “Current leakage and electrical shorts are due to metal migration between conductors and can be best described as a reverse plating of the copper or tin/lead conductors in the presence of ions, water, and electrical potential.”3 In simple terms, in the presence of moisture and electrical bias, metal ions are re-deposited between conductors, creating bridges or alternate electrical paths.3
Early on, tests were developed to detect the presence of free halides in inactivated flux chemistries. Standard qualitative tests, such as the silver chromate (IPC J-STD-004A) tests, detect the presence of free halide and suggest potential for corrosion. However, in the case of no-clean chemistries, potential corrosion may not represent actual field results in the final product, as no-clean chemistries are designed to remain after reflow as encapsulated and/or benign residues.
Qualitative methods, such as IPC J-STD-004A, can be ineffective for no-clean chemistries for other reasons:
- other activator components or activator packages (organic acids/bases) that have potential for corrosion;
- chemistries with halide may pass tests;
- chemistries without halide may fail the test.4
Today, most SMT assembly manufacturers have resolved to rely on rigorous simulation tests to verify that electrical reliability will not be compromised by corrosion, current leakage, or metal dendrite formation. These tests are referred to as surface insulation resistance (SIR) and electro migration (EM). Performed in high humidity and temperature conditions under an electrical bias, the tests require high insulation resistance (non-conductive) and negligible sign of corrosion or dendrite formation to pass.
Solder paste is reflowed onto a combed pattern and placed into a test chamber, tested without pre-cleaning for no-clean chemistries, where humidity and temperature are controlled according to the test and test method. See Table 1 for common test parameters and pass criteria.
It is not the purpose of this paper to detail the actual tests involved, but to recognize the difference in approach of these test methods.5 Qualitative tests like IPC J-STD-004A detect the presence of free halide in inactivated flux chemistry, which represents potential for corrosion. SIR and EM subject reflowed solder paste to high, but controlled, humidity and temperature conditions under an electrical bias. SIR and EM reflect actual field-like results, relating the effect of flux chemistry on corrosion.
Conclusion
Flux chemistries are generally comprised of certain categories of ingredients that have a tremendous affect on the function of the solder paste and the solder paste attributes. The primary function of the activator is to remove surface oxides, preparing the surfaces to be joined. Halide-containing activators provide exceptional oxide removal, but free (ionic) halide can affect shelf life, open stencil time, and reliability. These detrimental effects include corrosion, dendrite growth, and current leakage. As no-clean chemistries are designed to remain on the assembly after reflow, qualitative tests, such as the silver chromate test, may not effectively predict corrosion in the end product. SMT assembly manufacturers, as well as paste manufactures, focus heavily on the more rigorous simulation tests, such as SIR and EM, for product performance.
REFERENCES:
- Ning-Cheng Lee, Ph.D., Reflow Soldering Processes and Troubleshooting – SMT, BGA, CSP and Flip Chip Technologies.
- J. Augis, et al, “A Humidity Threshold for Conductive Anodic Filaments in Epoxy Glass Printed Wiring Boards,” Proceedings, 3rd International SAMPLE Electronics Conference, 1989, pp.1023-30.
- R. Michalkiewicz, J. Green, and S. Opperhauser, Surface Insulation Resistance Testing of Soldering Pastes and Fluxes, 2001.
- Indium Corporation Technical Bulletin.
- For further reference and detailed information concerning the actual tests, see IPC-TM-650, J-STD-004A.
Ronald C. Lasky, Ph.D., P.E., is a senior technologist at Indium Corporation and Instructional Professor at Dartmouth College, he blogs at www.indium.com/drlasky. Contact him at (603) 646-9197; ronlasky@aol.com. Edward Briggs is a technical support engineer at Indium Corporation. He earned a Green Belt Six-Sigma Certification from Dartmouth College. Contact him at (315) 381-7594; ebriggs@indium.com.