Solder paste is made with a combination of solder powder and a thick flux which is preblended to form a paste. This combination produces a thick material, which allows for easier deposition, typically through stencil printing or deposition . The paste forms a tacky consistency that prevents movement of components after placement on a printed circuit board (PCB) during SMT production. This results in high speed and volume throughput for production . In solder reflow applications, the solder paste is heated to an above liquidus temperature. At this temperature, flux is activated and will remove oxides from the surface of the leads, metal pads, and surface of the solder powder, allowing the solder powder to melt effectively and form solid metal joints [1,2].
Solder paste is made from a combination of solder powder and flux to achieve a homogenous creamy mixture for consistency in application. Solder powder has two important properties, the alloy type and the particle size [1,3]. These properties determine the time above liquidus, level of oxides, strength of the intermetallic bond, minimum aperture size, and the inner diameter for dispense printing. Flux is made of a combination of components such as rosins, activators, rheological additives, and solvents. The combination of these components determines what subcategory of flux it falls under, which dictates the rheological properties of the material and how the flux removes oxides [1,3]. This guide aims to provide information on the material properties of solder paste, its role in an SMT (surface mount technology) production line, and an overview of common defects.
Properties of Solder Paste
Solder powder can be composed of a variety of alloys, the most common being a combination of eutectic (tin and lead) and lead free, typically SAC305 (tin, silver 3.0, copper 0.5) . Both alloys are comprised mostly of tin which has a low melting point and high tensile and shear strength . Additional metals can also be added to further change the mechanical properties of the alloy.
For example, lead lowers the overall melting temperature and forms strong joints with other metals such as copper and aluminum which are often used for PCB pads or component leads [1,6]. These benefits are a reason as to why a sizable percentage of alloys for solder paste use a eutectic base. However, lead is considered both a health and environmental hazard so there has been a push in the industry to produce products that use less hazardous materials.
This has led to the emergence of lead-free alternatives, the most common of them being SAC305, which is comprised of tin, silver, and copper. Compared to eutectic solder, this SAC305 has a higher melting point. SAC305 is mostly made of tin, silver and copper are added to supplement the strength and lower melting temperature to improve the properties of the alloy. Copper reduces the melting point of the alloy and aids in the wetting of the molten solder while silver adds mechanical strength but is less ductile than lead .
These are only two of the most common alloys that are used in solder paste, however, there can be other alloys that have a combination of different metals and percentage levels to improve the quality of the solder joint [4,5]. Some of these metals include but are not limited to antimony, bismuth, indium, and nickel. It is important to be aware of any interactions the alloy selection can have with the base metals it is joining as this can impact the security of the metal joint . Selection depends on what properties are desired, electronic standards that need to be met, and what types of electronic device are being made . It is important to consider these factors when selecting an alloy that works best for your application.
Flux is a material in metallurgy that serves multiple functions and is a key component in metal joining and extractive metallurgy. Flux provides the functions of being a stabilizer, a flowing agent and/or a chemical cleaning agent. In solder paste, the main purpose of flux is to act as a cleaning agent .
Flux is a reducing agent that prepares the surface of the metal by removing oxides, debris, and grime . This prepares the surface by preventing oxidation of the metal surface and on the joining material and aids in the wetting of molten metal . Wetting is important for solder paste; it reduces the surface tension of molten metal and allows for easier flow during soldering process [7,8].
Several types of flux are used in electronics and typically fit into three categories: water soluble fluxes, no-clean fluxes, and traditional rosin fluxes [7-10]. Each of these categories has their own advantages and disadvantages and are used as a flux base in solder paste depending on the application [9-10].
Water Soluble Fluxes – This subtype has excellent soldering performance and has long endurance during the manufacturing process . This subtype tends to be highly active, so it is highly effective at removing oxide layers and rarely burns off during the soldering process . Due to this, it is necessary to clean after the soldering process as residue will be left behind. This residue, if left on the board, can cause contamination and corrosion of parts, however, it can be cleaned with water. Newer processes tend to avoid this subtype as extensive cleaning is needed to ensure that all the flux is removed and requires specialized equipment. The flux composition for this subtype tends to be the most active of the flux subtypes and are often organic acid based (OR) and sometimes inorganic acid based (IN) [1,9-10].
No-Clean Flux – No-cleans were developed to try to eliminate the amount of cleaning that is often required in other flux subtypes. These fluxes tend to be less active compared to the other two flux subtypes, which minimizes the amount of residue left on the board. Flux performance should be carefully selected because if the activity is too mild it will not thoroughly clean the surfaces for soldering. With no-clean fluxes cleaning is not “required” however, depending on the application it is being used for, cleaning can still be done to remove unsafe or undesirable residue on the boards. The flux composition for this subtype is often rosin based with lower activity level (RO) [1,9-11].
Rosin Flux – This subtype has excellent soldering performance and has long endurance during the manufacturing process. The flux composition for this subtype rosin based needs to have high level activators that can be rosin or resin based (RO or RE). The flux in this subtype is less aggressive than water soluble based fluxes and causes less corrosion. Rosin based flux can provide a barrier and trap ionic residues on the surface to prevent contamination on the board. The flux itself, however, can contaminate the equipment that is used during manufacturing. In harsh environments this subtype can cause failures in PCB boards that have been produced. For SMT production an additional step of washing is required, which is quite expensive to clean as it requires specialized solvents [9-11].
The metal in solder powder needs to be in the form of small particles for the material to be used in automated deposition . For solder powder this is achieved by atomization, which is achieved by taking molten solder and turning it into a spray of small liquid droplets. These droplets harden, leaving behind millions of small droplets of various particle sizes . These droplets are then sieved through assorted sizes of fine mesh which sorts them into different particle range sizes based off sphere diameter . This powder goes through a series of different sized meshes and the range of particles that are collected from each mesh is considered a 'type' [1,12]. These types are recognized as IPC standards and in documents can be referred to as particle size or mesh size. When selecting the particle size for application purposes a general rule to follow is the “5 ball” rule which ensures that an aperture can fit a minimum of 5 particles side to side [1,12-13]. The table below shows the calculated minimum aperture size for mesh sizes Type 1-Type 8 (T1-T8).
When the particle size is large it is more difficult to print finer features. In recent years there has been a push towards minimization of electronics, which in turn has caused the development of smaller particle sizes to accommodate the designs required at finer pitches [12-13]. When working with fine feature printing, having a smaller particle size ensures a higher capability of printing at finer features . As the particle size of solder powder decreases, the overall surface area of the solder powder in bulk increases, resulting in an increased amount of oxides present within a paste . A higher amount of oxides can increase the likelihood of some defects such as solder balling .
For the vast majority of SMT applications, solder pastes with Type 3 and Type 4 particle sizes are reliable, readily available, and robust which makes them the most common pastes available on the market [12-13].
Surface Mount Technology Production Line
As previously mentioned, solder paste is often used as a part of an SMT (surface mount technology) production line. These lines are used to produce PCBs rapidly and reliably . The SMT production line is comprised of five main pieces of equipment, which can be seen in the image below.
It is important to ensure that the base materials for this process such as surface mount components (SMC), PCBs, and solder pastes are in good condition and inspected before starting the production process. All systems should be set up for the intended PCB board design. This includes putting paste into the printing system, correct stencil placement, setting up programs for inspections, SMCs set up in the pick-and-place, and ensuring the correct reflow profile is selected [14-15]. Finally, it is important to ensure that the SMT production line sets the PCB in a fixed position during the entirety of the process. If a board is not fixed or in an incorrect position at any point during the assembly process it can cause major defects. Ensuring that these steps are properly taken can help mitigate any issues that can arise due to improper set-up.
Printers and Dispensers
Solder paste has a thick and creamy texture that allows for easy deposition onto PCBs utilizing a printing method. There are two main types of printing that are used with solder paste, stencil printing and deposition printing, each with their own advantages and disadvantages.
Stencil printing evolved from the screen-printing process, in which a stencil is made in a pattern of apertures matching a designed PCB out of a thin metal sheet. When paste is applied across the stencil it leaves behind paste where deposition is desired . Stencils are often treated or coated to allow for paste to roll smoothly across the surface and evenly distribute it to the desired location on the board. During production a PCB is set up with the stencil placed precisely on top. Paste is then deposited on one side of the board and a squeegee is used to wipe the paste across the stencil. Then, when the PCB is removed, the deposited paste from the stencil remains on top of the desired pad locations.
This process allows for precise volume of paste, high speed, high throughput, repeatability, and is the most common method of printing in PCB production . It is important to note that this method requires the PCB and stencil to be designed carefully, and during the design process stencils can go through multiple rounds of revisions to ensure the correct amount of paste is deposited on the PCB pads. Once the stencil is set up, this method can be very rapid and reliable, but it is fallible. The quality of the print can be affected by paste changes, damage to the stencil, incorrect printing settings, and incorrect positioning during printing. It is important before starting this printing process to inspect the condition of the stencil, squeegee blades, placement of the stencil, and paste quality to help prevent poor quality printing.
Solder paste formulation for deposition printing is slightly more liquidous compared to the solder paste developed for stencil printing. In deposition printing solder paste is stored in a syringe and the paste is deposited by forcing the paste through a needle or by jetting, allowing for precise amounts of solder paste to be dispensed [1,14]. With this method, syringes can easily be swapped out during printing making it easy to use multiple types of solder pastes for one board, which can be beneficial when working in a range of pitch sizes . This method offers a large amount of control and flexibility in how the paste is dispensed in different locations on the board. Compared to stencil printing, this method is much slower and more suited for smaller volumes of printing. As such, deposition printing is not widely used for full scale production .
Solder Paste Inspection
Solder paste inspection (SPI) is an automatic process that inspects the paste deposit quality on PCBs. This process inspects the paste’s appearance and the volume of the deposit on the PCB's metal pads . This step is to ensure that no defects occurred during the printing process so the boards can be cleaned, recovered, or disposed of before components are placed on the board.
Pick-and-Place Surface Mount
An SMT component placement system, often called a pick-and-place machine (P&P) is comprised of robotic machines that place SMCs onto printed pre-reflowed PCBs. These robotic machines use high precision in placing a wide range of electrical components onto the boards. These components are loaded along the front and back of the machine and are automatically fed into the system to be used as needed during the printing process [1,15].
Reflow ovens are machines that are used to reflow solder on a PCB with SMCs using controlled heat. This source of heat can be infrared, convection, or vapor phase. Reflow ovens are structured to have a long tunnel conveyor belt, which contains multiple heated zones [1,15]. Reflow ovens have multiple zones that have controlled temperature and speed. The settings that are recommended for each zone in the oven are known as a reflow profile. Typically, when solder paste is purchased, the manufacturer of the paste suggests a reflow profile to ensure that the material properly reflows. It is important when setting up the reflow profile that the temperatures do not exceed the rated temperature for the solder paste or associated components, otherwise it could cause damage to the PCB and SMCs during this process. The figure below shows an example of a thermal profile that is often provided by solder manufactures to input into a reflow oven.
Ramp to Soak
In this stage the board temperature climbs to the soak or dwell temperature. The purpose of this step is to get the assembly safely to the target soak temperature. This stage also allows for solvents to be outgassed before higher temperatures are reached [1,15]. Determining this ramp rate depends on desired process time and if there are heat sensitive components on the PCB.
This stage typically lasts around 60 to 120 seconds and is used to remove solder paste volatiles and start the activation of the flux . In addition, it ensures that the board and the paste have reached thermal equilibrium before moving to the reflow stage. If thermal equilibrium is not reached it can impact how the solder paste will behave and can cause defects during reflow [1,15].
Ramp to Peak
This step is a short ramp cycle to reach the reflowing temperature and only lasts for a few seconds.
In this stage the maximum temperature for reflow is reached and is known as the time above liquidus (TAL) or the amount of time that the solder needs to be in a liquidus state. This value is determined by the solder paste manufacturer. Having an incorrect temperature or time can result in defects and poor intermetallic bonds [1,15].
This is the final stage in a reflow oven. In this stage the PCB gradually cools and allows for the solder joints to form properly. If this stage is too quick it can cause breaking and cracking on components, however if it is too slow it can cause the intermetallic bonds to degrade. It is important to review the manufacturer’s specification for each solder paste, however, a common cooling rate is 4°C/s .
Automatic Optical Inspection
The last step of the SMT production line is an automatic optical inspection (AOI) that provides fast inspection of the assemblies on the PCBs to ensure that there are no defects present on the board . This process is typically only implemented for high volume productions as AOI machines are expensive and complicated to reprogram changes. This tool is used for monitoring and informs a user if there is a defect but typically does not inform the root cause of a problem. It is up to the user to determine the type of defect, the cause, and how it may be remedied [1,17].
Common Solder Paste Defects
Defects in solder paste can arise at any time due to changes in materials or improper preparation prior to application. This can cause many issues in production including, but not limited to, unexpected rework, delays, cost, etc. It is important to recognize the common solder paste defects that can occur before and after the reflow process and how they can be prevented beforehand .
Solder paste bleeding occurs when small clumps of solder paste are deposited next to pads after stencil printing .
Bridging occurs when the solder paste is deposited in between two different pads which then cause both pads to be connected .
An incomplete print occurs when solder paste does not completely go through the aperture and is improperly deposited on metal pads. This leaves small and incomplete deposits of paste on the pads .
Misalignment occurs when solder paste is deposited incorrectly and out of alignment in the printing process. The result leaves solder paste that is not correctly aligned to the center of the pads .
Peaking / Dog Ears
Peaking, also known as dog ears, occurs when solder paste is deposited in the correct position but there is a paste deposit with a peak on one side. This can cause issues in component placement and dislodgment of components during the reflow process .
Scooping / Scavenging
Scooping, also known as scavenging, occurs during the printing process when the center of paste that has been deposited on the pad has been scooped out and leaves a divot behind .
Slumping occurs when the paste spreads out after application and before components are placed. This can lead to paste spreading across pads and inadequate height of components. Once reflowed this can cause bridging and shorts to occur .
Bridging occurs when solder runs from one solder contact to a second contact point. After reflow, this solder connection can cause shorts to occur in a final product . Bridging that occurs post reflow has a similar appearance for both hot and cold slump. It is advised to inspect the board before reflow to first see if cold slumping has occurred. If it has not occurred, the bridging is likely due to hot slumping. This additional step can help find the root cause of the defect.
Cold Joints / Disturbed Joint / Grainy Joints
Cold Joints, otherwise known as disturbed joints or grainy joints, will have dull and rough appearance on the solder. The joint will have reduced strength and only a few intermetallic bonds .
Note: Solder paste with high lead content can also cause a dull appearance after reflowing
Dewetting occurs when metal pads get wet and then pull back to form droplets of solder on the surface. Dewetting gives the PCB surface a tinned appearance .
Note: It is important to review all these causes individually until the root case is discovered and resolved
Excess Solder on Fillet
Excess solder on a fillet occurs when bulbous convex joints completely obscure the leads due to the amount of solder .
EXCESS SOLDER ON FILLET
Graping / Cold Shoulder
Graping, also known as cold shoulder, occurs when dark, rough, non-reflective, small grainy deposits of paste appear and resemble bunches of grapes .
Head in Pillow
Head in pillow occurs when spheres from certain types of components, such as BGA or CSP, will not meld together with solder paste on the PCB pads . Both the solder sphere and deposit may fully melt, but the solder joint does not properly form.
HEAD IN PILLOW
Non-Wetting occurs when there is an insufficient solder fillet (concave surface of solder intersection of metal surfaces of a solder connection) .
Note: It is important to review all these causes individually until the root case is discovered and resolved
An open/insufficient fillet occurs when the bond with the solder between the lead and the pad is incomplete .
Solder balling occurs when tiny solder balls appear outside of metal pads on a PCB board. They can appear around the edge of the flux residue after reflowing, or they might be stuck around fine pitch lands .
Solder beading occurs when individual large solder balls appear next to components or metal pads and are most often found near discrete components .
Tombstoning occurs when chip type components stand on end after reflowing which is caused by unequal soldering on each end of the component. This defect most commonly occurs in the vapor phase of reflow .
Unmelted paste occurs when solder paste has not properly melted on components after the reflow cycle .
Note: A possible cause of this defect could be that the actual temperature of the reflow oven is higher/lower than what is being input to the machine. It is highly recommended that you pair a thermocouple to ensure that the oven is running at the expected temperatures.
Voiding occurs when ‘bubbles,’ which could be air or flux entrapment, appear within the joint. This defect is usually only found via x-ray or cross section inspection .
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