Solder Paste: Design Rules and Process Limits
Solder paste is a mixture of metal powder and flux, and both halves of that mixture have to be chosen for the board rather than picked from a catalogue. The alloy sets the temperature at which the joint forms and the mechanical behaviour of the metal afterwards. The powder size sets the smallest aperture that can be printed, and the flux sets what is left on the board and how the joint wets.
This article explains what each part of the choice controls, how the common alloys differ, and how the combination is matched to the product.
What The Alloy Decides
The alloy determines the melting range and therefore the profile, the strength and ductility of the joint, and its behaviour under thermal cycling. A eutectic alloy has a single melting point and solidifies cleanly, while a non eutectic one has a pasty range in which the joint is partly liquid, and a joint that is disturbed in that range will be rough and mechanically poor.
The alloy also sets the intermetallic that forms with the pad finish and with the component terminations, and therefore the way the joint ages. Joints that have to survive many thermal cycles are usually made with an alloy that is more compliant, even where a stiffer one would be easier to process. The mechanisms were described under PCB design and fabrication.
Common Alloys And Their Ranges
The tin lead eutectic alloy melts at 183 degrees Celsius and has been the reference for decades, and it is still used where the product is exempt from the restriction on lead, such as in some aerospace and defence equipment. Its advantages are a low process temperature, a wide process window and a joint that is easy to inspect.
The lead free alloys based on tin, silver and copper melt between about 217 and 221 degrees, with a pasty range of a few degrees depending on the exact composition. They are stronger and more creep resistant than the tin lead alloy but they require a higher process temperature, they wet less readily, and their joints look duller, which changes what an inspector expects to see. Other alloys add bismuth or antimony for specific purposes, and each of them changes the melting range in a way that has to be understood before it is used on a mixed assembly. The interactions between alloys are described under lead free versus leaded solder.
Powder Size And Printability
The metal in a paste is graded by the size of its particles, and the grades are numbered from type 3 to type 7, with the higher numbers being finer. A finer powder prints a smaller aperture and releases better from a thin stencil, and it also oxidises faster because it has more surface area per unit of volume, which shortens the working life and can increase voiding.
The choice follows the smallest aperture on the board. A type 3 powder suits a 0.5 millimetre pitch, a type 4 is used for 0.4 millimetre and fine work, and a type 5 for the finest pitches. Choosing a finer powder than necessary costs more and reduces the shelf life, while choosing a coarser one makes the fine apertures clog. The relationship with the stencil is described under manufacturable design guidelines.
The Flux And What It Leaves
The flux is classified by its activity and by whether it is designed to be cleaned. A rosin based flux leaves a residue that has to be removed for a high impedance circuit, while a no clean formulation leaves a residue that is intended to stay and that is qualified as electrically benign. A water soluble flux is the most active and must be cleaned, and leaving it in place is one of the fastest ways to produce a corrosion failure.
The activity also determines how the paste behaves on a difficult surface. An oxidised pad, a contaminated finish or a long storage period all need more activity, and a no clean flux that is designed for a clean board may not cope. The cleaning decision that follows is described under conformal coating and board protection.
Selecting For The Application
The selection starts with the product rather than with the paste. A consumer product assembled on a modern line uses a lead free alloy with a type 4 or 5 powder and a no clean flux. A product with a large thermal mass and limited peak temperature may need a lower melting alloy or a longer profile. A product that will be cleaned, coated and used in a humid environment needs a flux whose residue is compatible with the coating.
The second input is the assembly sequence. A paste with a high metal load gives a stronger joint and a lower void content but prints less readily; a paste with a lower load prints more easily and slumps more. A paste that will be printed and left for hours before reflow needs a longer tack time, while one used in a continuous line does not. Each of these is a trade and none of them can be judged from a datasheet alone. The sequence as a whole is described under PCBA development process.
Compatibility And Verification
A change of paste is a change of process. The print window, the profile, the residue and the inspection criteria all move, and the change has to be qualified with a first article inspection, a profile measurement and a check of the ionic contamination if the board will be coated. A substitution made in the middle of a production run is the source of a defect that no one can explain.
The qualification is best done with a test vehicle that carries the same aperture sizes, the same pad finishes and the same components as the product. Measuring the print volume, the joint appearance and the voiding on that vehicle gives a set of figures that can be compared with the previous paste, and it turns a subjective judgement into a decision. The standards that the joints are judged against are described under lead free versus leaded solder.
Additional Considerations for This Build
Practical attention to melting point pays for itself here, because it decides whether the finished board behaves as the drawing intended. Where the requirement is not stated on the fabrication drawing or in the assembly notes, the shop has to assume a default, and that default is rarely the value the design was simulated with. Stating melting point explicitly, together with the tolerance that applies, removes the assumption and keeps the result predictable from batch to batch.
Process Control and Verification
On a design of this kind, alloy is the item that decides how the rest of the board is arranged. The measurements that matter are the repeatable ones: conductor width and spacing, annular ring, finished hole size, plating thickness and surface finish are all verifiable on a coupon that travels with the panel. Running a first article through the same checks as the production panel confirms that the two agree, and that comparison is the cheapest form of process control available at prototype stage.
Bismuth, Antimony And Low Temperature Alloys
Alloys that contain bismuth melt well below the tin silver copper range, sometimes as low as 138 degrees, and they are used where a component cannot tolerate a high temperature or where a lower profile saves energy. Their weakness is that bismuth forms a low melting phase with the lead in a leaded component or in a leaded finish, and a joint that contains that phase can melt at a temperature below the service temperature of the product. That risk is the reason a bismuth alloy is used only after the whole assembly has been reviewed for lead, including the component terminations and the board finish.
Antimony is added to improve strength and creep resistance, and it is found in some high reliability alloys. It also raises the melting range slightly and changes the appearance of the joint, so the inspection criteria have to be adjusted. Neither addition is an improvement in general terms; each trades one property for another, and the trade is only worth making when the application demands it.
FAQ
Can a lead free paste be used on a leaded board? It can, and the result is a mixed alloy joint. The reliability of that joint has to be established by test rather than assumed, because the melting behaviour of the mixture is different from either alloy.
Does a finer powder always print better? It releases better from a small aperture, but it also oxidises faster and has a shorter working life, so it is chosen for the finest apertures rather than as a general improvement.
How long can paste be left on the stencil? That depends on the formulation and on the room, and it is measured in hours rather than days. Paste that has been on the stencil for a full shift should be discarded rather than returned to the jar.
