Solder paste

Last updated August 17, 2024

Solder paste is used in the manufacture of printed circuit boards to connect surface mount components to pads on the board. It is also possible to solder through-hole pin in paste components by printing solder paste in and over the holes. The sticky paste temporarily holds components in place; the board is then heated, melting the paste and forming a mechanical bond as well as an electrical connection. The paste is applied to the board by jet printing, stencil printing or syringe; then the components are put in place by a pick-and-place machine or by hand.

Use

A majority of the defects in circuit-board assembly are caused due to issues in the solder-paste printing process or due to defects in the solder paste. There are many different types of defects possible, e.g. too much solder, or the solder melts and connects too many wires (bridging), resulting in a short circuit. Insufficient amounts of paste result in incomplete circuits. Head-in-pillow defects, or incomplete coalescence of ball grid array (BGA) sphere and solder paste deposit, is a failure mode that has seen increased frequency since the transition to lead-free soldering. Often missed during inspection, a head-in-pillow (HIP) defect appears like a head resting on a pillow with a visible separation in the solder joint at the interface of the BGA sphere and reflowed paste deposit.^[1] 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.

When making PCBs (printed circuit boards), manufacturers often test the solder paste deposits using SPI (solder paste inspection). SPI systems measure the volume of the solder pads before the components are applied and the solder melted. SPI systems can reduce the incidence of solder-related defects to statistically insignificant amounts. Inline systems are manufactured by various companies such as Delvitech (Switzerland), Sinic-Tek (China), Koh Young (Korea), GOEPEL electronic (Germany), CyberOptics (US), Parmi (Korea) and Test Research, Inc. (Taiwan).^[2] Offline systems are manufactured by various companies such as by VisionMaster, Inc. (US) and Sinic-Tek (China).

Composition

A solder paste is essentially powdered solder suspended in flux paste. The tackiness of the flux holds components in place until the soldering reflow process melts the solder. As a result of environmental legislation, most solders today, including solder pastes, are made of lead-free alloys^{[ citation needed ]}.

Classification

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^{[ citation needed ]}.

Solder pastes are classified based on the particle size by IPC standard J-STD 005.^[3] The table below shows the classification type of a paste compared with the mesh size and particle size.^[4] Some suppliers use proprietary particle size descriptions, Henkel/Loctite descriptions are given for comparison.^[5]

Type designation [IPC]	Mesh size in lines-per-inch	Max. size in μm (no larger than)	Max. size in μm (less than 1% larger than)	Particle size in μm (80% min. between)	Avg. size in μm	Min. size in μm (10% max. less than)	Henkel Powder Description^[5]
Type 1			150	150-75		20
Type 2	-200/+325		75	75–45	60	20
Type 3	-325/+500		45	45–25	36	20	AGS
Type 4	-400/+635		38	38–20	31	20	DAP
Type 5	-500/+635	30	25	25–10		10	KBP
Type 6	-635	20	15	15–5		5
Type 7		15	11	11–2
Type 8		11	10	8–2

By flux

According to IPC standard J-STD-004 "Requirements for Soldering Fluxes", solder pastes are classified into three types based on the flux types:

Rosin based fluxes are made with rosin, a natural extract from pine trees. These fluxes can be cleaned if required after the soldering process using a solvent (potentially including chlorofluorocarbons) or saponifying flux remover.

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 designed to leave only small amounts of inert flux 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 reflow environment.

Properties of solder paste

In using solder paste for circuit assemblies, one needs to test and understand the various rheological properties of a solder paste.

Viscosity: The degree to which the material resists the tendency to flow. 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.

Thixotropic index: Solder paste is thixotropic, meaning that its viscosity changes with applied shear force (such as stirring or spreading). The thixotropic index is a measure of the viscosity of the solder paste at rest, compared to the viscosity of "worked" paste. Depending upon the formulation of the paste, it may be very important to stir the paste before use, to ensure that the viscosity is appropriate for proper application. When solder paste is moved by the squeegee on the stencil, the physical stress applied to the paste causes the viscosity to drop, allowing the paste to flow easily through the apertures on the stencil. When the stress on the paste is removed, it regains its viscosity, preventing it from flowing on the circuit board.

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.

Tack: Tack is the property of a solder paste to hold a component after the component had been placed by the placement machine. Hence, tack life is the critical property of solder pastes. It is defined as the length of time that solder paste can remain exposed to the atmosphere without a significant change in tack properties. A solder paste with long tack life is more likely to provide the user with a consistent and robust printing process.

Response-to-pause: Response-to-pause (RTP) is measured by the difference in volume of solder paste deposition as a function of number of prints and pause time. A large variation in the print volume after a pause is unacceptable as this causes end of line defects such as shorts or opens. A good solder paste shows less variation in the volume of the prints after pause. However, another may show large variations and also an overall decreasing trend in volume.

Use

Solder paste is typically used in a stencil-printing process by a solder paste printer,^[6] 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 ejected onto 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 a complete reflow soldering process.

The paste manufacturer will suggest a suitable reflow temperature profile to suit their individual paste. The main requirement is a gentle rise in temperature to prevent explosive expansion (which can cause "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.

For a good soldered joint, the proper amount of solder paste must be used. Too much paste may result in a short circuit; too little may result in poor electrical connection or physical strength. Although solder paste typically contains around 90% metal in solids by weight, the volume of the soldered joint is only about half that of the solder paste applied.^[7] This is due to the presence of flux and other non-metallic agents in the paste, and the lower density of the metal particles when suspended in the paste as compared to the final, solid alloy.

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.

Storage

Solder paste must be refrigerated when transported and stored in an airtight container at a temperature between 0-10 °C. It should be warmed to room temperature for use.

Recently, new solder pastes have been introduced that remain stable at 26.5 °C for one year and at 40 °C for one month.^[8]

Exposure of the solder particles, in their raw powder form, to air causes them to oxidize, so exposure should be minimized.

Evaluation

The main reason why evaluation of solder paste is necessary, is because 50-90% of all defects result from printing problems. Hence, paste evaluation is critical.

This procedure is quite thorough, yet minimizes the amount of testing required to differentiate between excellent and poor solder pastes. If multiple solder pastes are evaluated, the procedure can be used to eliminate the poor pastes from their poor printing quality. Further testing, such as solder reflow performance, solder joint quality, and reliability testing can then be performed on the solder paste finalists.

Concerns

The main concerns about solder paste are:

It can dry out on the stencil if kept out for too long.
It may be toxic.
It is expensive and waste has to be minimized.

These three concerns helped to spawn three enclosed systems for printing.

DEK ProFlow
MPM Rheometric Pump Head
Fuji Cross Flow

Related Research Articles

Solder is a fusible metal alloy used to create a permanent bond between metal workpieces. Solder is melted in order to wet the parts of the joint, where it adheres to and connects the pieces after cooling. Metals or alloys suitable for use as solder should have a lower melting point than the pieces to be joined. The solder should also be resistant to oxidative and corrosive effects that would degrade the joint over time. Solder used in making electrical connections also needs to have favorable electrical characteristics.

A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits. BGA packages are used to permanently mount devices such as microprocessors. A BGA can provide more interconnection pins than can be put on a dual in-line or flat package. The whole bottom surface of the device can be used, instead of just the perimeter. The traces connecting the package's leads to the wires or balls which connect the die to package are also on average shorter than with a perimeter-only type, leading to better performance at high speeds.

Surface-mount technology (SMT), originally called planar mounting, is a method in which the electrical components are mounted directly onto the surface of a printed circuit board (PCB). An electrical component mounted in this manner is referred to as a surface-mount device (SMD). In industry, this approach has largely replaced the through-hole technology construction method of fitting components, in large part because SMT allows for increased manufacturing automation which reduces cost and improves quality. It also allows for more components to fit on a given area of substrate. Both technologies can be used on the same board, with the through-hole technology often used for components not suitable for surface mounting such as large transformers and heat-sinked power semiconductors.

In metallurgy, a flux is a chemical reducing agent, flowing agent, or purifying agent. Fluxes may have more than one function at a time. They are used in both extractive metallurgy and metal joining.

Thixotropy is a time-dependent shear thinning property. Certain gels or fluids that are thick or viscous under static conditions will flow over time when shaken, agitated, shear-stressed, or otherwise stressed. They then take a fixed time to return to a more viscous state. Some non-Newtonian pseudoplastic fluids show a time-dependent change in viscosity; the longer the fluid undergoes shear stress, the lower its viscosity. A thixotropic fluid is a fluid which takes a finite time to attain equilibrium viscosity when introduced to a steep change in shear rate. Some thixotropic fluids return to a gel state almost instantly, such as ketchup, and are called pseudoplastic fluids. Others such as yogurt take much longer and can become nearly solid. Many gels and colloids are thixotropic materials, exhibiting a stable form at rest but becoming fluid when agitated. Thixotropy arises because particles or structured solutes require time to organize.

A reflow oven is a machine used primarily for reflow soldering of surface mount electronic components to printed circuit boards (PCBs).

Wave soldering is a bulk soldering process used for the manufacturing of printed circuit boards. The circuit board is passed over a pan of molten solder in which a pump produces an upwelling of solder that looks like a standing wave. As the circuit board makes contact with this wave, the components become soldered to the board. Wave soldering is used for both through-hole printed circuit assemblies, and surface mount. In the latter case, the components are glued onto the surface of a printed circuit board (PCB) by placement equipment, before being run through the molten solder wave. Wave soldering is mainly used in soldering of through hole components.

Reflow soldering is a process in which a solder paste is used to temporarily attach anywhere from one to thousands of tiny electrical components to their contact pads, after which the entire assembly is subjected to controlled heat. The solder paste reflows in a molten state, creating permanent solder joints. Heating may be accomplished by passing the assembly through a reflow oven, under an infrared lamp, or by soldering individual joints with a hot air pencil.

<span class="mw-page-title-main">Rework (electronics)</span> Refinishing operation of an electronic printed circuit board assembly

In electronics, rework is the repair or refinish of a printed circuit board (PCB) assembly, usually involving desoldering and re-soldering of surface-mounted electronic components (SMD). Mass processing techniques are not applicable to single device repair or replacement, and specialized manual techniques by expert personnel using appropriate equipment are required to replace defective components; area array packages such as ball grid array (BGA) devices particularly require expertise and appropriate tools. A hot air gun or hot air station is used to heat devices and melt solder, and specialised tools are used to pick up and position often tiny components. A rework station is a place to do this work—the tools and supplies for this work, typically on a workbench. Other kinds of rework require other tools.

Selective soldering is the process of selectively soldering components to printed circuit boards and molded modules that could be damaged by the heat of a reflow oven or wave soldering in a traditional surface-mount technology (SMT) or through-hole technology assembly processes. This usually follows an SMT oven reflow process; parts to be selectively soldered are usually surrounded by parts that have been previously soldered in a surface-mount reflow process, and the selective-solder process must be sufficiently precise to avoid damaging them.

Flat no-leads packages such as quad-flat no-leads (QFN) and dual-flat no-leads (DFN) physically and electrically connect integrated circuits to printed circuit boards. Flat no-leads, also known as micro leadframe (MLF) and SON, is a surface-mount technology, one of several package technologies that connect ICs to the surfaces of PCBs without through-holes. Flat no-lead is a near chip scale plastic encapsulated package made with a planar copper lead frame substrate. Perimeter lands on the package bottom provide electrical connections to the PCB. Flat no-lead packages usually, but not always, include an exposed thermally conductive pad to improve heat transfer out of the IC. Heat transfer can be further facilitated by metal vias in the thermal pad. The QFN package is similar to the quad-flat package (QFP), and a ball grid array (BGA).

<span class="mw-page-title-main">Automated optical inspection</span> System for visual inspection of printed circuit boards by a computerized system

Automated optical inspection (AOI) is an automated visual inspection of printed circuit board (PCB) manufacture where a camera autonomously scans the device under test for both catastrophic failure and quality defects. It is commonly used in the manufacturing process because it is a non-contact test method. It is implemented at many stages through the manufacturing process including bare board inspection, solder paste inspection (SPI), pre-reflow and post-re-flow as well as other stages.

Bead probe technology (BPT) is technique used to provide electrical access to printed circuit board (PCB) circuitry for performing in-circuit testing (ICT). It makes use of small beads of solder placed onto the board's traces to allow measuring and controlling of the signals using a test probe. This permits test access to boards on which standard ICT test pads are not feasible due to space constraints.

Thick-film technology is used to produce electronic devices/modules such as surface mount devices modules, hybrid integrated circuits, heating elements, integrated passive devices and sensors. Main manufacturing technique is screen printing (stenciling), which in addition to use in manufacturing electronic devices can also be used for various graphic reproduction targets. It became one of the key manufacturing/miniaturisation techniques of electronic devices/modules during 1950s. Typical film thickness – manufactured with thick film manufacturing processes for electronic devices – is 0.0001 to 0.1 mm.

A thermal profile is a complex set of time-temperature data typically associated with the measurement of thermal temperatures in an oven. The thermal profile is often measured along a variety of dimensions such as slope, soak, time above liquidus (TAL), and peak.

<span class="mw-page-title-main">Soldering</span> Process of joining metal pieces with heated filler metal

Soldering is a process of joining two metal surfaces together using a filler metal called solder. The soldering process involves heating the surfaces to be joined and melting the solder, which is then allowed to cool and solidify, creating a strong and durable joint.

<span class="mw-page-title-main">Graping</span> Phenomenon in soldering

Graping is a phenomenon marked by the appearance of unreflowed solder particles on top of the solder mass. The solder that is partially coalesced resembles a cluster of grapes, hence the derivation of the phenomenon’s name.

Stencil printing is the process of depositing solder paste on the printed wiring boards (PWBs) to establish electrical connections. It is immediately followed by the component placement stage. The equipment and materials used in this stage are a stencil, solder paste, and a printer.

In the assembly of integrated circuit packages to printed circuit boards, a head-in-pillow defect, also called ball-and-socket, is a failure of the soldering process. For example, in the case of a ball grid array (BGA) package, the pre-deposited solder ball on the package and the solder paste applied to the circuit board may both melt, but the melted solder does not join. A cross-section through the failed joint shows a distinct boundary between the solder ball on the part and the solder paste on the circuit board, rather like a section through a head resting on a pillow.

Digital image correlation analyses have applications in material property characterization, displacement measurement, and strain mapping. As such, DIC is becoming an increasingly popular tool when evaluating the thermo-mechanical behavior of electronic components and systems.

References

↑ Alpha (2010-03-15) [September 2009]. "Reducing Head in Pillow Defects - Head in pillow defects: causes and potential solutions". 3. Archived from the original on 2013-12-03. Retrieved 2018-06-18.
↑ Robles Consée, Marisa (2015). "Marktübersicht SPI-Systeme - Den optimalen Lotpasten-Druckprozess im Visier" (PDF). Productronic (in German). 2015 (7): 42–45. 450pr0715. Archived (PDF) from the original on 2018-06-18. Retrieved 2018-06-18.
↑ Solder Paste Task Group (January 1995). "J-STD-005 Requirements for Soldering Pastes". Arlington, Virginia: Electronic Industries Alliance (EIA) and IPC.
↑ Tarr, Martin (2006-10-03). "Solder paste basics". Online postgraduate courses for the electronics industry. UK: University of Bolton. Archived from the original on 2010-11-12. Retrieved 2010-10-03.
1 2 "Technical Data Sheet LOCTITE HF 212" (PDF). Henkel. June 2016.
↑ "Solder-Paste Printer". Yamaha Motor Co., Ltd.
↑ "Solder Volumes for Through-Hole Reflow-Compatible Connectors" (PDF). Tyco Electronics Corporation. Archived (PDF) from the original on 2018-06-18. Retrieved 2016-08-29.
↑ Wilding, Ian (2016-02-08). "First-Ever Temperature Stable Solder Paste Unveiled - Major Development in Solder Paste Formulation Set to Change Market Paradigms". Henkel Electronics . The Electronics Group of Henkel. Archived (PDF) from the original on 2016-03-02. Retrieved 2021-02-25.