Solder paste
Encyclopedia
Solder paste sometimes refers to soldering flux
that does not contain solder.
Solder paste (or solder cream) is used to connect the leads of integrated chip packages to attachment points (lands) in the circuit patterns on a printed circuit board
. The paste is typically applied to the lands using a stencil to "print" the paste, although other methods, like dispensing, are also used. Typically, solder paste accounts for 0.05% of a circuit board's final cost.
Solder paste is used with surface mount devices.
A majority of the defects circuit-board assembly are caused due to issues in solder-paste printing process or due to defects in the solder paste. An electronics manufacturer needs experience with the printing process, specifically the paste characteristics, to avoid costly re-work on the assemblies. The paste's physical characteristics, like viscosity and flux levels, need to be monitored periodically by performing in-house tests.
suspended in a thick medium called flux
. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection. The paste is a gray, putty-like material. The composition of the solder paste varies, depending upon its intended use. For example, when soldering plastic component packages to a FR-4
glass epoxy circuit board, the solder compositions used are eutectic
Sn-Pb (63 percent tin
, 37 percent lead
) or SAC alloys (tin/silver
/copper
, named for the elemental symbols Sn/Ag/Cu). If one needs high tensile and shear strength, tin-antimony
(Sn/Sb) alloys might be used with such a board. Generally, solder pastes are made of a tin
-lead
alloy, with possibly a third metal alloyed, although environmental protection legislation is forcing a move to lead-free solder.
Solder paste is thixotropic, meaning that its viscosity
changes over time with applied shear force (e.g. stirring). The thixotropic index is a measure of the viscosity of the solder paste at rest, compared to "worked" paste. Depending upon the formulation of the paste, it may be very important to stir the paste before it is used, to ensure that the viscosity is appropriate for proper application.
Solder pastes are classified based on the particle size by JEDEC
standard J-STD 005. The table below shows the classification type of a paste compared with the mesh size and particle size.
Rosin based pastes are made of rosin, a natural extract from pine trees. These fluxes need to be cleaned after the soldering process using chlorofluorocarbon
s (CFCs). Due to the ban on CFCs, rosin fluxes are no longer predominant.
Water soluble fluxes are made up of organic materials and glycol bases. There is a wide variety of cleaning agents for these fluxes.
A no-clean flux is made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. However, these pastes need a very clean assembly environment and may need an inert re-flow environment.
properties of a solder paste.
Viscosity
Slump
Working life
process, in which paste is deposited over a stainless steel
or polyester
mask to create the desired pattern on a printed circuit board
. The paste may be dispensed pneumatically
, by pin transfer (where a grid of pins is dipped in solder paste and then applied to the board), or by jet printing (where the paste is sprayed on the pads through nozzles, like an inkjet printer
).
As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold the components while the assembly passes through the various manufacturing processes, perhaps moved around the factory.
Printing is followed by pre-heating and reflow (melting).
The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste; however, one can expend too much energy on this. The main requirement is a gentle rise in temperature to prevent explosive expansion ("solder balling"), yet activate the flux. Thereafter, the solder melts. The time in this area is known as Time Above Liquidus. A reasonably rapid cool-down period is required after this time.
A good tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.
As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards (e.g. J-std, JIS, IPC) exist to measure the safety of the residues left behind.
In most countries, "no-clean" solder pastes are the most common; in the United States
, water-soluble pastes (which have compulsory cleaning requirements) are common.
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
that does not contain solder.
Solder paste (or solder cream) is used to connect the leads of integrated chip packages to attachment points (lands) in the circuit patterns on a printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
. The paste is typically applied to the lands using a stencil to "print" the paste, although other methods, like dispensing, are also used. Typically, solder paste accounts for 0.05% of a circuit board's final cost.
Solder paste is used with surface mount devices.
A majority of the defects circuit-board assembly are caused due to issues in solder-paste printing process or due to defects in the solder paste. An electronics manufacturer needs experience with the printing process, specifically the paste characteristics, to avoid costly re-work on the assemblies. The paste's physical characteristics, like viscosity and flux levels, need to be monitored periodically by performing in-house tests.
Composition
A solder paste is essentially powdered metal solderSolder
Solder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...
suspended in a thick medium called flux
Flux (metallurgy)
In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
. Flux is added to act as a temporary adhesive, holding the components until the soldering process melts the solder and makes a stronger physical connection. The paste is a gray, putty-like material. The composition of the solder paste varies, depending upon its intended use. For example, when soldering plastic component packages to a FR-4
FR-4
FR-4 is a grade designation assigned to glass-reinforced epoxy laminate sheets, tubes, rods and printed circuit boards . FR-4 is a composite material composed of woven fiberglass cloth with an epoxy resin binder that is flame resistant .FR-4 glass epoxy is a popular and versatile high-pressure...
glass epoxy circuit board, the solder compositions used are eutectic
Eutectic system
A eutectic system is a mixture of chemical compounds or elements that has a single chemical composition that solidifies at a lower temperature than any other composition. This composition is known as the eutectic composition and the temperature is known as the eutectic temperature...
Sn-Pb (63 percent tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
, 37 percent lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
) or SAC alloys (tin/silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
/copper
Copper
Copper is a chemical element with the symbol Cu and atomic number 29. It is a ductile metal with very high thermal and electrical conductivity. Pure copper is soft and malleable; an exposed surface has a reddish-orange tarnish...
, named for the elemental symbols Sn/Ag/Cu). If one needs high tensile and shear strength, tin-antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
(Sn/Sb) alloys might be used with such a board. Generally, solder pastes are made of a tin
Tin
Tin is a chemical element with the symbol Sn and atomic number 50. It is a main group metal in group 14 of the periodic table. Tin shows chemical similarity to both neighboring group 14 elements, germanium and lead and has two possible oxidation states, +2 and the slightly more stable +4...
-lead
Lead
Lead is a main-group element in the carbon group with the symbol Pb and atomic number 82. Lead is a soft, malleable poor metal. It is also counted as one of the heavy metals. Metallic lead has a bluish-white color after being freshly cut, but it soon tarnishes to a dull grayish color when exposed...
alloy, with possibly a third metal alloyed, although environmental protection legislation is forcing a move to lead-free solder.
Solder paste is thixotropic, meaning that its viscosity
Viscosity
Viscosity is a measure of the resistance of a fluid which is being deformed by either shear or tensile stress. In everyday terms , viscosity is "thickness" or "internal friction". Thus, water is "thin", having a lower viscosity, while honey is "thick", having a higher viscosity...
changes over time with applied shear force (e.g. stirring). The thixotropic index is a measure of the viscosity of the solder paste at rest, compared to "worked" paste. Depending upon the formulation of the paste, it may be very important to stir the paste before it is used, to ensure that the viscosity is appropriate for proper application.
By size
The size and shape of the metal particles in the solder paste determines how well the paste will "print". A solder ball is spherical in shape; this helps in reducing surface oxidation and ensures good joint formation with the adjoining particles. Irregular particle sizes are not used, as they tend to clog the stencil, causing printing defects. To produce a quality solder joint, it's very important for the spheres of metal to be very regular in size and have a low level of oxidation.Solder pastes are classified based on the particle size by JEDEC
JEDEC
The JEDEC Solid State Technology Association, formerly known as the Joint Electron Devices Engineering Council , is an independent semiconductor engineering trade organization and standardization body...
standard J-STD 005. The table below shows the classification type of a paste compared with the mesh size and particle size.
| Type designation [JEDEC] | Mesh size in lines-per-inch | Max. size (no larger than) |
Max. size (less than 1% larger than) |
Particle size in um (80% min. between) |
Avg. size in um |
Avg. size in um (10% max. less than) |
|---|---|---|---|---|---|---|
| Type 1 | 150 | 150-75 | 20 | |||
| Type 2 | -200/+325 | 75 | 75–45 | 60 | 20 | |
| Type 3 | -325/+500 | 45 | 45–20 | 36 | 20 | |
| Type 4 | -400/+635 | 38 | 38–20 | 31 | 20 | |
| Type 5 | -500 | 30 | 25 | 25–10 | 10 | |
| Type 6 | -635 | 20 | 15 | 15–5 | 5 | |
| Type 7 | 15 | 11 | 11–2 | |||
| Type 8 | 11 | 10 | 8–2 |
By flux
According to JEDEC standard J-STD-004 "Requirements for Soldering Fluxes", solder pastes are classified into three types based on the flux types:Rosin based pastes are made of rosin, a natural extract from pine trees. These fluxes need to be cleaned after the soldering process using chlorofluorocarbon
Chlorofluorocarbon
A chlorofluorocarbon is an organic compound that contains carbon, chlorine, and fluorine, produced as a volatile derivative of methane and ethane. A common subclass are the hydrochlorofluorocarbons , which contain hydrogen, as well. They are also commonly known by the DuPont trade name Freon...
s (CFCs). Due to the ban on CFCs, rosin fluxes are no longer predominant.
Water soluble fluxes are made up of organic materials and glycol bases. There is a wide variety of cleaning agents for these fluxes.
A no-clean flux is made with resins and various levels of solid residues. No-clean pastes save not only cleaning costs, but also capital expenditures and floor space. However, these pastes need a very clean assembly environment and may need an inert re-flow environment.
Properties of solder paste
In using solder paste for circuit assemblies, one needs to test and understand the various rheologicalRheometry
Rheometry generically refers to the experimental techniques used to determine the rheological properties of materials, that is the quantitative and qualitative relationships between deformations and stresses and their derivatives.The choice of the adequate experimental technique depends on the...
properties of a solder paste.
Viscosity
- The degree to which the material resists the tendency to flow. In this case, varying viscosities of solder paste are desired at different levels of shearing force. Such a material is called thixotropic. When solder paste is moved by the squeegeeSqueegeeA squeegee, squilgee or sometimes squimjim, is an onomatopoeically named tool with a flat, smooth rubber blade, used to remove or control the flow of liquid on a flat surface...
on the stencil, the physical stress applied to the paste causes the viscosity to break down, thinning the paste and helping it flow easily through the apertures on the stencil. When the stress on the paste is removed, it regains it shape, preventing it from flowing on the circuit board. The viscosity for a particular paste is available from the manufacturer's catalog; in-house testing is sometimes needed to judge the remaining usability of solder paste after a period of use.
Slump
- The characteristic of a material's tendency to spread after application. Theoretically, the paste's sidewalls are perfectly straight after the paste is deposited on the circuit board, and it will remain like that until the part placement. If the paste has a high slump value, it might deviate from the expected behavior, as now the paste's sidewalls are not perfectly straight. A paste's slump should be minimized, as slump creates the risk of forming solder bridges between two adjacent lands, creating a short circuit.
Working life
- The amount of time solder paste can stay on a stencil without affecting its printing properties. The paste manufacturer provides this value.
Use
Solder paste is typically used in a screen-printingScreen-printing
Screen printing is a printing technique that uses a woven mesh to support an ink-blocking stencil. The attached stencil forms open areas of mesh that transfer ink or other printable materials which can be pressed through the mesh as a sharp-edged image onto a substrate...
process, in which paste is deposited over a stainless steel
Stainless steel
In metallurgy, stainless steel, also known as inox steel or inox from French "inoxydable", is defined as a steel alloy with a minimum of 10.5 or 11% chromium content by mass....
or polyester
Polyester
Polyester is a category of polymers which contain the ester functional group in their main chain. Although there are many polyesters, the term "polyester" as a specific material most commonly refers to polyethylene terephthalate...
mask to create the desired pattern on a printed circuit board
Printed circuit board
A printed circuit board, or PCB, is used to mechanically support and electrically connect electronic components using conductive pathways, tracks or signal traces etched from copper sheets laminated onto a non-conductive substrate. It is also referred to as printed wiring board or etched wiring...
. The paste may be dispensed pneumatically
Pneumatics
Pneumatics is a branch of technology, which deals with the study and application of use of pressurized gas to effect mechanical motion.Pneumatic systems are extensively used in industry, where factories are commonly plumbed with compressed air or compressed inert gases...
, by pin transfer (where a grid of pins is dipped in solder paste and then applied to the board), or by jet printing (where the paste is sprayed on the pads through nozzles, like an inkjet printer
Inkjet printer
An inkjet printer is a type of computer printer that creates a digital image by propelling droplets of ink onto paper. Inkjet printers are the most commonly used type of printer and range from small inexpensive consumer models to very large professional machines that can cost up to thousands of...
).
As well as forming the solder joint itself, the paste carrier/flux must have sufficient tackiness to hold the components while the assembly passes through the various manufacturing processes, perhaps moved around the factory.
Printing is followed by pre-heating and reflow (melting).
The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste; however, one can expend too much energy on this. The main requirement is a gentle rise in temperature to prevent explosive expansion ("solder balling"), yet activate the flux. Thereafter, the solder melts. The time in this area is known as Time Above Liquidus. A reasonably rapid cool-down period is required after this time.
A good tin/lead solder joint will be shiny and relatively concave. This will be less so with lead-free solders.
As with all fluxes used in electronics, residues left behind may be harmful to the circuit, and standards (e.g. J-std, JIS, IPC) exist to measure the safety of the residues left behind.
In most countries, "no-clean" solder pastes are the most common; in the United States
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
, water-soluble pastes (which have compulsory cleaning requirements) are common.
Storage
Solder paste should be stored in an airtight container at low, but above freezing, temperatures. It should be warmed to room temperature for use. Exposure of the solder particles, in their raw powder form, to air causes them to oxidize, so exposure should be minimized.See also
- fluxFlux (metallurgy)In metallurgy, a flux , is a chemical cleaning agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time...
- Helping hand (tool)Helping hand (tool)A helping hand, also known as a third hand or X-tra Hands, is a type of jig used in soldering and craftwork. A commonly produced version consists of a weighted base, arms ending in alligator clips, and optionally a magnifying glass, held together by flexible joints...
- solderSolderSolder is a fusible metal alloy used to join together metal workpieces and having a melting point below that of the workpiece.Soft solder is what is most often thought of when solder or soldering are mentioned and it typically has a melting range of . It is commonly used in electronics and...
- solderingSolderingSoldering is a process in which two or more metal items are joined together by melting and flowing a filler metal into the joint, the filler metal having a lower melting point than the workpiece...
- wave solderingWave solderingWave soldering is a large-scale soldering process by which electronic components are soldered to a printed circuit board to form an electronic assembly. The name is derived from the use of waves of molten solder to attach metal components to the PCB...
- reflow solderingReflow solderingReflow soldering is a process in which a solder paste is used to temporarily attach one or several electrical components to their contact pads, after which the entire assembly is subjected to controlled heat, which melts the solder, permanently connecting the joint...
- Restriction of Hazardous Substances DirectiveRestriction of Hazardous Substances DirectiveThe Directive on the restriction of the use of certain hazardous substances in electrical and electronic equipment 2002/95/EC was adopted in February 2003 by the European Union. The RoHS directive took effect on 1 July 2006, and is required to be enforced and become law in each member state...
(RoHS) - Non-Newtonian fluidNon-Newtonian fluidA non-Newtonian fluid is a fluid whose flow properties differ in any way from those of Newtonian fluids. Most commonly the viscosity of non-Newtonian fluids is not independent of shear rate or shear rate history...