What is in a Solder Paste Flux?
The specification that defines solder paste (and flux) is
J-Std-004A (Requirements for Soldering Fluxes) and
J-Std-001D (Requirements for Soldered Electrical and Electronic Assemblies).
|Rosin or Resin?||Activator Levels How acidic is the paste?||Halides (a percent by weight)||Flux Type||Flux Designator|
|Rosins are "RO"||Low||0%||L0||ROL0|
|>0.5% - 2% (5000ppm - 20,000ppm)||M1||ROM1|
|Resins are "RE"||Low||0%||L0||REL0|
|>0.5% - 2% (5000ppm - 20,000ppm)||M1||REM1|
What is the difference between a rosin (RO) and a resin (RE)?
Rosins are the pulp and sap from pine trees. There are a lot of differences in exactly what is in these and it depends on geography. How the Rosin is harvested makes a huge difference. Some are from dead tree stumps, others live trees, the type of pine tree varies (furs, long needle, short needle etc). With so much variation the melting temperature range, the clarity, the color, the impurities and many other differences exist. The formulator picks specific types to get the color of the residue, the way it melts or burns, the thickening or slumping properties and other factors. In many cases 2 different Rosins are blended to get better properties. The biggest issue with Rosins is the impurities and variations found in a natural material
Resins come in 2 flavors. Some are purified Rosins (Modified Rosins) – where some of the impurities are removed and others are Synthetic. The Synthetic Resins are more consistent; the impurities are either zero or introduced deliberately to get a positive result. Their promise is consistency and control over performance. The problem is that natural materials have properties that depend on these impurities along with the range of chemical variations nature provides. In the past, Synthetic Resins often fell short in these properties, they have been too pure, and lacking is robustness. Modified Rosins (Resins) can often eliminate the impurities that cause problems and keep the best properties.
What is an Activator?
An activator is also called an acid. Common activators include carbolic acid (found in tree rosin), Malonic acid, an acetic acid and many others. Each of these chemicals is weak acids. Acids are great because they are strippers. They strip the dirt and oxidation off the metal surfaces, and then when the solder melts; it wets to these surfaces and forms a strong, clean joint. Without activators areas of the metallization do not solder, cold joints and grainy joints occur.
Different activators are active over different temperature ranges and for different amounts of time and are better on different types of surfaces. One way of looking at this is to have a strong activator that strips off the oxides and dirt at a low temperature and another that keeps the surfaces clean at a higher temperature, with others targeting specific types of surfaces (like Copper, Nickel, and Organic coatings).
Once the acid has done its job, it may end up in several ways: Some “unzip” at higher temperatures and are carried away in the exhaust – they do not remain on the board. Others break down and are acidic in the raw paste at reflow temperature, but neutral in the residue. This can occur because they are sealed into the flux residue, or they decompose. Still others are “active” and can (Potentially) continue to eat away at the metallization, or create conductive paths between pads. The types of activators watched by the specification bodies are Halides and Halogens
The activation level is coded as:
L = Low or no flux/flux residue activity-------------- “0%”
M = Moderate flux/flux residue activity -------------“<0.5%”
H = High flux/flux residue activity --------------------“<2%”
What are Halide and Halogen and what is the difference?
Halogen and normally white powders. They are (normally) salts made from strong a group of metals; the most common are Fluoride, chloride and bromide. Halides are the same materials, but in a different form. These are ionic fluoride (F−), chloride (Cl−), and bromide (Br−) and reactive. They are great at stabilizing viscosities, stripping metals and generally helpful in the reflow process. They are also active after reflow and on the watch list form the environmentalist.
There are 2 ways to give a Halide percentage. The first is by reviewing the formula and adding them up, the second is by testing the ionic activity. The direct measurement normally returns a much lower result. The industry specification is ROHS, Restriction of Hazardous Substances Directive. There are many company specifications which are the same or tougher. The actual spec calls for <900ppm of chloride (Cl−), and <900ppm of bromide (Br−) and a cumulative of <1500ppm
Because of 1) the numerous company requirements and the 2) definition of <1500ppm as “Zero” and 3) the difference between formula percentages and the testing percentages - confusion is the rule at the extremes
The activation level is coded as:
L = Low or no flux/flux residue activity ----------------“0%” ----------------(this is <1500ppm)
M = Moderate flux/flux residue activity ----------------“<0.5%” ------------(this is 5000ppm)
H = High flux/flux residue activity ----------------------“<2%” --------------(This is 20,000ppm)
Within the Rosin (RO) Activity Level (L,M and H) there are Halide levels (0,and 1). So a Halide level at zero is a “0” and a higher Halide level is a “1”.
So there exist 4 pastes one may see:L0, L1, M0 and M1. In theory, an H0 and H1 exist, but it is unlikely one will see these in electronics. No Cleans are L0s and occasionally L1s. RMAs and water soluble pastes are often higher – but those residues are cleaned off the boards and never have a chance to cause reliability problems. These can become problems for companies applying “green” standards.
What are common Solvents and how should I thin solder paste?
Common solvents are Terpineol and Glycol. Alcohol is rarely used – it causes the paste to dry out. Rosins are the sap and pulp with the oils extracts. These oils are turpentine’s, Terpineol and the like. Adding these back in thins the rosin (and Resins). Normally a very small amount will thin the solder paste dramatically. Solvent additions also affect the ability of the paste to sit on the stencil and print after a delay, the drying out of the paste on the stencil and board. It is also hard to control – a drop or two can do little to change the paste, then another drop shift it dramatically. It is hard to control.
The best way may be to add flux rather than a solvent. Both ways have potential issues. Adding flux changes the metal loading. A little may never be noticed, but too much means the printed metal is reduced in every joint – also the slump and tack will change. Still the difference between a dispensable solder paste and a printable solder paste in normally the viscosity caused by flux additions. A printable will normally have 89-90% metal and a dispensable 85-87% metal.
What does the thickener or tackifier do?
Thickeners and Tackifier are common additions in solder paste. There are a number of things occurring but 2 stand out. In printing, the solder paste should liquefy when pushed by the squeegee, but solidify at rest. In this way it prints well, but stays where you print it and does not slump. A thickener does this. It changes the Yield Point in the viscosity curve. The yield point is the point where the material gives way and flows. You can feel this if you put a spatula into a jar of paste and stir. At first is it difficult, but as you force the paste, it becomes easier – the paste “relaxes” and it takes less force. The squeegee is doing the same thing on a much smaller amount of paste (right at the point of the cut. If it “flows like a dream” and prints “like bricks” the thickener is tuned in to your process. A Tackifier is often the same additive, but can be different. It modified the tact or the ability to hold a component in place. The worst pastes for tact have been water soluble – They print like soap (because they were close to …soap) and components slide around. Tackifier solves this problem.
These are normally materials are long polymer chains and they are not volatized so they stay on the reflowed board in the form of residue. Because smaller amounts of residue are better – very small additions of these materials are desirable. It is balanced between residue, printing, and tact.