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STEP 3: Solder Materials By Peter Biocca, Kester
December 31, 1969 |Estimated reading time: 8 minutes
Soldering is the most critical part of the assembly process; without good solder joints, product functionality and reliability cannot be guaranteed. Through the years, soldering principles have remained the same. This article is a return to the basics of solder-material selection in a lead-free world.
By Peter Biocca, Kester
The fundamental rule of soldering is unchanged - create a bond using flux to remove oxides and prepare surfaces to be joined with a low-melting alloy. Lead-free soldering principles also are the same as leaded; however, the physical properties of these solders are different - wetting behavior is slower and the cosmetics sometimes differ. Achieving good solder joints using lead-free alloys has required manufacturers to revisit each process for optimization. In addition to higher heat needed to melt some lead-free solders, different fluxes must be used to reduce surface tension to promote good spread and wicking. Lead-free solder joint cosmetics also differ slightly from leaded, requiring training to recognize the difference between an acceptable joint and a poor one.
Soldering was revisited in the late-1980s with the introduction of no-clean fluxes, which require tighter process controls to achieve complete wetting. With lead-free soldering, equipment, materials, components, and boards also must be lead-free-process capable. All processes are affected - from hand-soldering to reflow, wave, and selective soldering to tinning.
Metals and Oxides
Before soldering can occur, oxides must be removed completely. There are numerous types of oxidation that form over metals, all of which prevent solder wetting. Some metals oxidize more readily than others; some form oxide bonds that are more tenacious. The less-oxidized parts are, the easier is to solder them together. Preserving solderability in assemblies is accomplished using various methods:
- tinning with more solderable finishes, such as pure tin;
- using organic surface preservatives like copper OSP;
- applying plated-solderability preservative, such as gold over nickel;
- handling boards and components by their edges, and using gloves or finger cots;
- storing boards/components in dry, low-humidity, and sulfur-free areas.
With lead-free, control over oxidation has become critical because many lead-free solders have lower spread than eutectic tin/lead (Sn63Pb37). Some assemblers that did not experience reduced wetting with leaded solders have revisited parts storage and handling. Good rotation practices, such as first-in/first-out (FIFO) must be stressed to keep solderability high.
Fluxes
Fluxes are essential to accomplish good wetting. In some cases, hydrogen gas is used as a reducing agent; however, this is impractical for most assembly processes. In most situations, a chemical or blend of chemicals is used to remove or reduce the surfaces to be bonded into the metal state. Soldering can only be accomplished if surfaces are oxide-free. If oxides are present after the reaction with flux, solder will not flow and large contact angles will result; good bonding cannot be guaranteed. With lead-free solders, larger contact angles can result because wetting is reduced slightly on some surfaces (Figure 1).
Figure 1. Solder wetting and non-wetting behaviors.
Fluxes are a blend of various ingredients that de-oxidize parts to be joined. Activators and acids are active ingredients that react with oxides. They can be mild organic acids or organic halides. Fluxes must meet certain flux classes, as defined by IPC; the amount of these activators is controlled to avoid corrosion issues and ensure their complete removal, if water-washable. With solder pastes, flux is blended with a high weight percentage of solder powder. The typical formulation of a solder paste flux can be:
- resins - 60%
- activators and acids - 5%
- solvents - 25%
- gelling agents - 5%
- wetting agents - 5%
In liquid fluxes, activators are dissolved in a carrier - alcohol, or water for VOC-free fluxes. In cored solder, activators are dissolved in a resinous blend of chemicals, ranging from gum rosin to synthetic resins. In tacky or gel fluxes, the activator is blended into a high-viscosity hydrocarbon liquid.
Solders
Solders are a combination of various metals that form an alloy to wet metal surfaces. They tend to melt at lower temperatures than the metals of which they are composed. When melted on certain cleaned metals, solder spreads, forming an intermetallic bond. The bond is a fusion of metals being soldered and the solder alloy. Figure 2 shows a typical micrograph of an intermetallic bond.
Figure 2. Intermetallic bond.
Intermetallic bonding will vary in thickness, depending on metals being soldered. A thin bond is sufficient for joint reliability. A thicker intermetallic bond created by overheating can cause joint embrittlement because the bond line has a higher tensile strength than the solder. Once a solder joint has been created, it is important not to rework it. Repeated melting will increase the thickness of the bond line, and could lead to brittle fractures.
Figure 3. Solder finishes on Sn63Pb37, proprietary lead-free solder A*, proprietary lead-free solder B**, and SAC 305.
Traditionally, Sn63Pb37 has been used in leaded soldering applications, but several lead-free alloy types have emerged. The most common of which is Sn96.5Ag3.0Cu0.5 (SAC 305), which has a melting point of about 217º-220°C. With hand, wave, and selective soldering applications, tin/copper-based solders can be used. These alloys melt at about 227°C. SAC 305 tends to result in a dull surface, while some tin/copper-based solders with additives of nickel, bismuth, or germanium result in brighter joints (Figure 3).
Hand Soldering
The process of hand soldering has received much attention with the lead-free transition. Operators accustomed to tin/lead regularly complain about lead-free solder’s lack of wetting, or the occurrence of cold solder joints, partly due to the fact that lead-free alloys flow more slowly and leave duller surface finishes. The main cause for these issues is related to process modifications required to work with these solders. With lead-free hand soldering, reliable joints are achievable, but several factors must be understood. Variables in hand soldering are choosing the correct tip geometry, increasing tip temperature to 700ºF or more, using the correct flux percentage in the wire, increasing contact time slightly, using a properly tinned tip, avoiding the use of additional liquid flux, and replacing tips regularly. Lead-free soldering is less forgiving than leaded; if these variables are not properly addressed, poor soldering will result in a loss of production yields.
Due to its higher tin content, lead-free solder tends to erode soldering-iron tips faster, which can reduce tip life. To increase tip life, turn off or reduce the tip temperature when not soldering, tin the tip before finishing, and avoid using abrasive cleaning tools.
Reflow Soldering
The reflow soldering process involves several operations, each of which must be optimized to ensure a good first-pass yield. Stencil printing, paste selection, component placement, and the oven process must be optimized. The reflow profile must be suited to the solder paste that has been selected. Lead-free soldering requires that components and boards be lead-free-process capable. Moisture-sensitivity levels also may change; it is worth reviewing IPC-020C for all hermetically sealed surface mount device (SMD) parts being soldered.
The solder paste selection process is important because various properties of solder pastes such as printability, slump behavior, flux activity, residue probe-ability, and cosmetics will affect success.
Solder-powder diameter is typically Type III (25-45 µm); however, there is a trend toward finer powders as line spacing gets smaller. Finer powders have a higher risk of oxidation during use, and the shelf life of these products may be reduced. Proper storage and handling will optimize shelf life of all solder pastes.
Typical defects with reflow soldering include bridges, lack of wetting, solder balls, mid-chip balling, residue-removal difficulties, voids, blowholes, and de-wetting. These defects can be prevented if all process parameters in paste selection, printing, placement, and reflow operations are selected carefully.
Figure 5. Thermal profile for lead-free reflow soldering.
The thermal profile can also help minimize defects, and is critical in understanding the importance of soak zone and maximum peak temperature. Time above liquidus (TAL) and cool-down zone each can contribute to soldering defects. A typical lead-free thermal profile used for SAC 305 pastes is shown in Figure 4. The preheat zone equalizes board and component temperatures and removes volatile solvents. TAL is important to ensure adequate melting of the solder powder and create the necessary solder wicking and spread. TAL should be at least 40 seconds to achieve this. When it exceeds 75 seconds, thicker intermetallic layers can be formed, leading to solder joint embrittlement.
Wave Soldering
The principles of wave soldering have not changed much. The key elements are described below. With lead-free wave soldering, these elements become more important to access during the validation process:
- solder alloy: SAC or tin/copper-based;
- flux type: no-clean or water washable;
- flux volume: applied with spray or foam;
- preheat of assembly and flux: with or without convection;
- solder-to-board contact thickness: usually ½ to ¾ thickness of board;
- Solder contact time: longer with lead-free;
- Solder contact width: more with lead-free.
A properly optimized wave solder process should not cause soldering defects. Lead-free solders have increased the incidence of certain types of defects due to their physical properties. These can be prevented with control over solderability, and carefully chosen soldering materials. Flux selection is important because the flux must be designed for lead-free wave soldering, and must be able to withstand longer dwell times and promote hole fill with solders that flow less readily than tin/lead.
With the advent of VOC-free liquid fluxes for wave soldering, the use of convection heating has gained acceptance. Proper preheating of the assembly is needed to remove the solvent water before any soldering can occur. Inadequate preheating will result in a lack of hole fill and blowholes. If water-washable fluxes are used, flux activity is better, but ensuring that boards are well-cleaned after soldering is important. Any remaining flux by-products will reduce the surface insulation of the board, leading to corrosion or dendritic growth. This potentially can cause electrical failures. Ionic contamination testing is recommended to verify board cleanliness. Common wave solder defects are lack of hole fill, poor wetting, bridges, flagging, blowholes, de-wetting, and solder-mask blistering.
Conclusion
Soldering is not magic; it is the science of metallurgical bonding. Removing oxide layers using a flux, heating the parts to be bonded, melting the solder, and the wetting of metals to be joined will result in a good intermetallic bond. Each soldering process has unique requirements to satisfy this. If each process is optimized carefully, then soldering defects are few and production yields are good - ensuring long-term solder joint reliability.
* K100 lead-free solder, Kester.** K100LD lead-free solder, Kester.
Peter Biocca, senior market development engineer, Kester, may be contacted at (972) 390-1197; e-mail: pbiocca@kester.com.