Solder paste is a material that helping soldering electronic components on the PCB board. In the SMT Assembly factory, they print solder paste on the pads of bare PCB by stencil, then pick and place components on the solder paste, this sticky paste will hold the components to pass reflow oven , after heated, melting the paste and forming a mechanical bond as well as an electrical parts,A PCBA will come out with electronic function .
Classification of solder paste
1st: According to the type of environmental protection, solder paste can be divided into two categories: leaded and lead-free.
- Leaded solder paste is made of a mixture of flux paste and solder powder. The main components are Sn and Pb. The most commonly used leaded solder paste is Sn63/Pb37.
- Lead-free solder paste refers to that with a lead content that reaches the EU standard of less than 1000ppm, and it is mainly composed of tin, silver and copper.
2nd. According to the temperatures required for use, it can be divided into: high temperature, normal temperature, and low temperature solder paste.
Generally speaking, solder paste with a melting point lower than 138°C is called low temperature. Its composition is mainly tin-bismuth alloy. The melting point of normal temperature solder paste is between 150-250℃, and the melting point of high temperature solder paste is above 250℃.
3rd. According to its need for cleaning, it can be divided into: environmental cleaner solder paste and disposable solder paste.
- Disposable solder paste is a uniform mixture of solder particles of no-clean flux and also contains some additives, which is an ideal feature for the production of SMT.
- The environmental cleaner solder paste uses water-soluble flux. After soldering, the residue reacts with water and may cause a short circuit after getting damp, so it must be cleaned.
4th.According to the activity, it can be divided into: high RA/medium RMA/low R type solder paste.
- Low R type: Contains natural rosin, no other added catalysts.
- Medium RMA type: In addition to natural rosin, a catalyst based on halogen is added.
- High RA: The addition of catalyst is extremely strong. The welding effect is good, but corrosion is high.
The role of solder paste ingredients
Solder paste is mainly composed of flux and solder powder.
(1) The main components and functions of flux:
- ACTIVATION: This component mainly plays a role in removing oxidizing substances on the surface of PCB copper film pads and parts welding parts, and at the same time, has the effect of reducing the surface tension of tin and lead.
- THIXOTROPIC: This ingredient is mainly used to adjust the viscosity of solder paste and printing performance, and plays a role in preventing tailing and sticking during printing.
- RESINS: Mainly increases the adhesion of solder paste, and also protects and prevents PCB from re-oxidation after soldering; this component plays an important role in fixing the parts.
- SOLVENT: This ingredient is the solvent of the flux, which plays a role in uniformity and the stirring process of the solder paste, and has a certain impact on the life of the solder paste as well.
(2) Solder powder:
Solder powder, also known as tin powder, is mainly composed of tin-lead, tin-bismuth, or tin-silver-copper alloy, and the general ratios are SN63/PB37, SN42BI58, SN96.5CU0.5AG3.0 and SN99CU0.7AG0.3. In summary, the relevant characteristics and quality requirements of tin powder are as follows:
- The particle shape of the tin powder has a great influence on the working performance of solder paste:
- The most important point is that the particle size distribution of tin powder is required to be uniform. Here we will talk about the distribution ratio of tin powder particle size. In domestic solder powder or solder paste manufacturing, we often use the distribution ratio to measure the tin powder uniformity. Taking 25~45μm tin powder as an example, it is usually required that the graduation ratio of 35μm particles is about 60%, and the parts below 35μm and above account for 20% each.
- In addition, the shape of tin powder particles is required to be relatively regular. According to the “General Specification for Tin-Lead Paste Solder” (SJ/T 11186-1998) in the Electronic Industry Standard of the People’s Republic of China, the relevant regulations are as follows: “It is spherical, but allows the maximum ratio of the major axis to the minor axis to be 1.5. If the user and the manufacturer reach an agreement, it can also be alloy powder of other shapes.”In actual work, it is usually required that the ratio of the long and short axis of the tin powder particles is generally below 1.2.
- If the basic requirements of A-1 and A-2 above cannot be met, the printing, spotting and soldering effects of the solder paste will likely be affected during its use.
- The ratio of tin powder to flux in various solder pastes is also different. When choosing solder paste, you should choose according to the products produced, the production process, the precision of the soldering components, and the requirements for the soldering effect.
- According to the relevant regulations in the “General Specification for Tin-Lead Paste Solder” (SJ/T 11186-1998) in the Electronic Industry Standard of the People’s Republic of China, “The percentage (mass) content of alloy powder in the solder paste should be 65%-96%, the deviation between the actual measured value of the alloy powder percentage (mass) content and the predetermined value in the order form is not more than ±1%”. In actual use, the tin powder content of the selected solder paste is usually about 90%, that is, the ratio of tin powder to flux is roughly 90:10.
- The common printing process mostly uses tin paste with a tin powder content of 89-91.5%.
- When using the needle point-injection process, use solder paste with a tin powder content of 84-87%.
- Reflow soldering requires that the device pins are soldered firmly, the solder joints are full and smooth, and there is a 1/3 to 2/3 height solder climb in the height direction of the pat (resistance container part). The metal alloy in the solder paste has a certain effect on the thickness of the solder after reflow soldering (that is, the fullness of the solder joint).
- C. The “low oxidation degree” of tin powder is also a very important quality requirement, and a problem that should be paid attention to during the production or storage of tin powder. If you do not pay attention to this problem, use tin powder with a higher oxidation degree. The solder paste will seriously affect the quality of soldering during the soldering process.
What is solder
Solder is a kind of fusible metal whose melting point is lower than that of the metal being welded. When the solder is melted, it can wet the surface of the metal to be welded under the condition that the metal is not melted, and an alloy layer is formed at the touch surface which connects with the metal to be welded. In general electronic product assembly, this is commonly known as solder.
Conditions for commonly used solders:
- The melting point of the solder is lower than the part to be welded.
- It is easy to connect with the welded object and has a certain ability to suppress stress.
- It must have good electrical conductivity.
- A fast crystallization rate is required.
Common types of solder
According to its different melting points, solder can be divided into hard solder and soft solder; according to the different compositions, it can be divided into tin-lead solder, silver solder, copper solder, etc. In the soldering process, tin-lead alloy solder is generally used.
1) Tin-lead solder is a commonly used tin-lead alloy which is mainly composed of tin and lead, and also contains trace metal components such as antimony.
The main purposes of tin-lead solder paste: It is widely used in the electronic industry for soft soldering, and for certain applications in the radiator and hardware industries, such as wave soldering, dip soldering and other precision soldering, as well as special welding processes, spraying, electroplating, etc. Anti-oxidation solder bars, produced by a special process of tempering and refining, feature unique high anti-oxidation performance, less scum than ordinary solder, less loss, good fluidity, strong solderability, uniform solder joints, brightness, etc.
Tin-lead solder strip
Tin-lead solder standard: GB/T8012-2000/GB/T3131-2001
2) Eutectic solder refers to tin-lead solder that reaches a eutectic composition. The alloy composition is 61.9% tin and 38.1% lead. In practical applications, solder containing 60% tin and 40% lead is generally called eutectic solder. Among the alloys of tin and lead, except for pure tin, pure copper and eutectic components which are melted at a single temperature, other alloys are melted over a temperature range, so eutectic solder has the best performance among tin-lead solders.
Eutectic solders: Two or more metal alloys with the lowest melting point. When heated, the eutectic alloy changes directly from solid to liquid without going through the plasticity stage.
The shape of commonly used solder:
Solder is often processed and shaped according to the specified size when it is used, and there are many kinds of sheet, block, rod, ribbon and wire.
- Wire-shaped solder: Usually called solder wire, or with rosin in the center, called rosin-cored wire, commonly used in manual soldering. The outer diameter of the rosin-cored welding wire usually includes 0.5mm, 0.6mm, 0.8mm, 1.0mm, 1.2mm, 1.6mm, 2.0mm, 2.3mm, 3.0mm and other specifications.
- Flake solder: Often used for welding silicon wafers and other flake welding parts.
- Ribbon solder: Often used in automatic assembly production lines. An automatic welding machine is used to punch a section from the ribbon-shaped solder for soldering to improve production efficiency.
- Solder paste: Made by mixing solder and flux together. When soldering, the solder paste is first applied to the printed circuit board and then soldered. It has been widely used in the automatic placement process.
Solder paste storage and use
1.Preservation method
The storage of the solder paste should be controlled at 1-10℃; the service life of the solder paste is 6 months (unopened); it should not be placed in the sun.
2.How to use (before opening)
The temperature of the solder paste must be raised to the ambient temperature (25±2°C) before opening, and the temperature recovery time is about 3-4 hours. It is forbidden to use other heaters to increase the temperature instantly. Fully stir after the ambient temperature is reached. The mixing time is 1-3 minutes, depending on the type of mixer.
3.How to use (after opening)
- Add about 2/3 of the solder paste to the stencil, and try to keep the amount not more than 1 can on the stencil.
- Depending on the production speed, the solder paste on the stencil should be added multiple times in small amounts to maintain the quality of the solder paste.
- Solder paste that has not been used up that day should not be placed together with unused solder paste, but should be stored in another container. It is recommended that the solder paste be used up within 24 hours at room temperature after opening.
- When using the next day, you should first use newly opened solder paste, then mix the unused solder paste and the new solder paste at a ratio of 1:2, and add them multiple times in small amounts.
- After the solder paste is printed on the substrate, it is recommended to place the parts into the reflow furnace within 4-6 hours to complete the solder.
- Please scrape the solder paste from the steel plate and put it into the solder paste tank before changing the wire for more than 1 hour.
- Due to dust in the air and other pollution, please follow the “step 4” method after the solder paste is continuously printed for 24 hours, in order to ensure product quality.
- To ensure printing quality, it is recommended to manually wipe the openings on both sides of the steel plate every 4 hours.
- Maintain the indoor temperature between 22-28℃ and humidity RH30-60% for the best working environment.
- To wipe off wrongly printed substrate, it is recommended to use industrial alcohol or industrial cleaning agent.
Automatic high precision solder paste printing machine
Lead-free solder paste
Now we will discuss the composition of lead-free solder paste and the best alloy composition.
Lead-free solder paste is mainly composed of tin/silver/copper, where silver and copper are used to replace the original lead.
The best alloy composition in the tin/silver/copper system is 95.4Sn/3.1Ag/1.5Cu, which has good strength, fatigue resistance and plasticity. However, it should be noted that the lowest melting temperature that the tin/silver/copper system can reach is 216~217°C, which is too high to be suitable for circuit board applications under the current SMT structure (melting temperature below 215°C is considered a de facto standard).
All in all, the alloy composition of the tin/silver/copper system containing 0.5~1.5% Cu and 3.0~3.1% Ag has quite good physical and mechanical properties. Quite speaking, the cost of 95.4Sn/3.1Ag/1.5Cu is lower than those alloys with high silver content,
such as 93.6Sn/4.7Ag/1.7Cu and 95.4Sn/4.1Ag/0.5Cu. In some cases, higher silver content may reduce certain properties.
Set solder paste reflow temperature curve
The correct temperature profile will ensure high-quality solder joints.
- Test method
In a printed circuit board (PCB) assembly using surface mount components, an optimized reflow temperature profile is one of the most important factors to obtain high-quality solder joints. The temperature curve is a function of the temperature applied to the circuit assembly versus time. When drawing on the Cartesian plane, at any given time during the reflow process, the temperature at a specific point on the PCB forms a curve. Several parameters affect the shape of the curve, the most important of which is the conveyor speed and the temperature setting of each zone. The belt speed determines the duration of the board’s exposure to the temperature set in each zone. Increasing the duration allows more time for the circuit assembly to approach the temperature setting of that zone. The sum of the duration spent in each zone determines the total processing time.
The temperature setting of each zone affects the temperature rise rate of the PCB, and a high temperature produces a larger difference between the temperature of the PCB and the zone. Increasing the set temperature of the zone allows the board to reach a given temperature faster. Therefore, a graph must be made to determine the temperature curve of the PCB. Next is the outline of this step to generate and optimize graphics.
Before starting the curve step, the following equipment and auxiliary tools are needed: temperature profiler, thermocouple, tool for attaching thermocouple to PCB, and solder paste parameter table. A temperature curve accessory kit can be purchased from most major electronic tool suppliers. This kit makes it easy to create curves because it contains all the necessary accessories (except the curve meter itself).
Many reflow soldering machines and even some smaller, inexpensive countertop furnaces include an on-board thermometer. Thermometers are generally divided into two categories. Real-time thermometers instantly transmit temperature/time data and make graphs, while the other type of thermometer samples and stores data, then uploads it to the computer.
The thermocouple must be long enough and able to withstand typical furnace temperatures. Generally, smaller diameter thermocouples have small thermal mass and quick response, resulting in more accurate results.
There are several ways to attach the thermocouple to the PCB. The best way is to use high temperature solder such as silver/tin alloy with the smallest possible solder joints.
Another acceptable method is fast, easy and accurate enough for most applications. Use a small amount of thermal compound (also called thermal grease or thermal grease) spots to cover the thermocouple, and then use high temperature tape (such as Kapton) to stick it.
A third way to attach thermocouples is to use high-temperature adhesives, such as cyanoacrylate adhesives. This method is generally not as reliable as others.
The attachment position should also be well-selected. It is usually best to attach the thermocouple tip between the PCB pad and the corresponding component pin or metal end.
The solder paste characteristic parameter table is also necessary, and the information it contains is crucial to the temperature profile, such as the desired duration of the temperature profile, solder paste activity temperature, alloy melting point, and the desired maximum reflow temperature.
Before you start, you must have a basic understanding of the ideal temperature profile. In theory, the ideal curve consists of four parts or sections. The first three sections are heated and the last section is cooled. The more temperature zones of the furnace, the more accurate the profile of the temperature curve can be. Most solder pastes can be successfully reflowed using four basic temperature zones.
The preheating zone, also called the ramp zone, is used to raise the temperature of the PCB from the ambient temperature to the required active temperature. In this area, the temperature of the product continuously rises at a rate of no more than 2~5°C per second. If the temperature rises too fast, it will cause some defects, such as micro-cracks in ceramic capacitors. If the temperature rises too slowly, the solder paste will also heat slowly, and there will not be enough time for the PCB to reach the active temperature. The preheating zone of the furnace generally occupies 25~33% of the length of the entire heating channel.
The active area, sometimes called the drying or soaking area, generally occupies 33-50% of the heating channel. It has two functions. The first is to sense the temperature of the PCB at a fairly stable temperature, allowing components of different qualities to blend and reducing their considerable temperature difference. The second function is to allow the flux to be activated and volatile substances to evaporate from the solder paste. The general active temperature range is 120~150°C. If the temperature of the active zone is set too high, the flux will not have enough time to activate, and the temperature curve will be an upwardly increasing slope. Although some solder paste manufacturers allow slight temperature increase during activation, the ideal curve requires a fairly stable temperature, so that the temperature of the PCB is equal at the beginning and the end of the active zone. Some furnaces on the market cannot maintain a flat active temperature curve. Choosing a furnace that can maintain a flat active temperature curve will improve solderability and give users a larger processing window.
The recirculation zone is sometimes called the peak zone or the last heating zone. The function of this zone is to increase the temperature of the PCB assembly from the active temperature to the recommended peak temperature. The active temperature is always a little lower than the melting point of the alloy, and the peak temperature is always at the melting point. The typical peak temperature range is 205~230°C. Setting the temperature in this zone too high will cause the temperature rise slope to exceed 2~5°C per second, or reach the reflux peak temperature higher than recommended. This situation may cause excessive curling, delamination or burning of the PCB, and damage the integrity of the components.
Nowadays, the most commonly used alloy is Sn63/Pb37. This ratio of tin and lead makes the alloy eutectic. Eutectic alloys are alloys that melt at a specific temperature. Non-eutectic alloys have a melting range, not a melting point, and are sometimes called plastic state. All the examples described in this article refer to eutectic tin/lead because it is widely used and the melting point of this alloy is 183°C. The ideal cooling zone curve should be a mirror image relationship with the reflux zone curve. The closer to this mirror image relationship, the closer the structure of the solder joint to solid state, the higher the quality of the solder joint and the better the integrity of the joint.
The first parameter to consider when making the temperature curve is the speed setting of the conveyor belt, which will determine the time that the PCB spends in the heating channel. Typical solder paste manufacturing factory parameters require a heating curve of 3 to 4 minutes. Divide the total heating channel length by the total heating temperature sensing time, which is the accurate conveyor speed. For example, when the solder paste requires four minutes of heating time, using six feet of heating channel length, you can calculate this as: 6 feet ÷ 4 minutes = 1.5 feet per minute = 18 inches per minute.
Next, you must decide the temperature settings of each zone. It is important to understand that the actual zone temperature is not necessarily the displayed temperature of the zone. The displayed temperature only represents the temperature of the thermocouple in the zone. If the thermocouple is closer to the heating source, the displayed temperature will be higher than the interval temperature. The closer the thermocouple is to the direct channel of the PCB, the more the displayed temperature will reflect the actual zone temperature. It is wise to consult the furnace manufacturer to understand the relationship between the clearly displayed temperature and the actual zone temperature. This article will consider the interval temperature rather than the display temperature. Table 1 lists the interval temperature used for typical PCB assembly reflow.
After the temperature is determined, it must be input to the furnace controller. Look at the other parameters in the manual that need to be adjusted. These parameters include cooling fan speed, forced air impulse, and inert gas flow. Once all the parameters are entered, start the machine, and when the furnace stabilizes (that is, all actual displayed temperatures are close to meeting the set parameters), the curve can be started. The next step is to put the PCB on the conveyor belt and trigger the thermometer to start recording data. For convenience, some thermometers include a trigger function to automatically start the thermometer at a relatively low temperature, which is typically slightly higher than the human body temperature of 37°C (98.6°F). For example, the automatic trigger of 38°C (100°F) allows the thermometer to start working almost when the PCB is put on the conveyor belt and into the furnace, so that the thermocouple will not be triggered by mistake when it is handled by the human.
- Analysis of test results
First, it must be verified that the total time from the ambient temperature to the peak temperature of the reflux is compatible with the desired residence time of the heating curve. If it is too long, increase the conveyor speed proportionally; if it is too short, the opposite is true.
Choose the curve that best matches the actual shape of the graph. The deviation from left to right (process sequence) should be considered. For example, if there is a difference between the preheating and recirculation zones, first adjust the difference in the preheating zone correctly. Generally, it is best to adjust one parameter at a time and run before making further adjustments. This is because a change in a given zone will also affect the results of subsequent zones. We also recommend that novices make relatively small adjustments. Once you have gained experience on a particular furnace, you will have a better “feel” to make more drastic adjustments.
When the final graph is as close as possible to the desired graph, the furnace parameters should be recorded or stored for later use. Although this process is slow and laborious at first, it can eventually gain proficiency and speed, resulting in high-efficiency production of high-quality PCBs.
Solder paste printing
Process equipment
In the solder paste printing process, the printer is the key to achieving the desired printing quality. The screen printing machines available today are divided into two main types: laboratory and production. Each type has a further classification, because each company hopes to get different performance levels from laboratory and production type printing presses. For example, a company’s research and development department (R&D) uses a laboratory type to make product prototypes, while production uses another type. Also, production requirements may vary greatly, depending on the output. Because laser cutting equipment is impossible to classify, it is best to choose a screen printing machine that is compatible with the desired application.
In manual or semi-automatic printing machines, the solder paste is manually placed on the template/screen, and the squeegee is at the other end of the template. In automatic printers, solder paste is automatically dispensed. During the printing process, the printing squeegee presses down on the template so that the bottom surface of the template contacts the top surface of the circuit board. When the squeegee walks through the entire length of the pattern area being corroded, the solder paste is printed on the pads through the openings on the template/screen.
After the solder paste has been deposited, the screen is snapped off immediately after the squeegee and returns to its original place. This separation or disengagement distance is determined by the equipment design, about 0.020″~0.040″. Disengagement distance and squeegee pressure are two important equipment-related variables to achieve good printing quality.
If it is not disconnected, this process is called on-contact printing. When using an all-metal template and squeegee, use contact printing. Off-contact printing is used for flexible metal screens.
Squeegee type
The wear, pressure and hardness of the squeegee determine the printing quality and should be carefully monitored. For acceptable printing quality, the edges of the squeegee should be sharp and straight. Low squeegee pressure causes omissions and rough edges, while high or very soft squeegees will cause smeared printing and may even damage the squeegee and template or screen. Excessive pressure also tends to dig out the solder paste from the wide openings, causing insufficient solder fillets.
There are two common types of scrapers: rubber or polyurethane scrapers and metal scrapers. When using a rubber scraper, use one with a 70-90 durometer hardness. When using overly high pressure, the solder paste that penetrates into the bottom of the template may cause solder bridges, requiring frequent bottom wiping. In order to prevent bottom penetration, the opening of the pad must provide a gasketing function during printing. This depends on the roughness of the template opening wall.
Metal scrapers are also commonly used. With the use of more closely spaced components, the number of metal scrapers is increasing. They are made of stainless steel or brass, have a flat blade shape, and use a printing angle of 30~45°. Some scrapers are coated with lubricating material. Because they use lower pressure, they will not dig out the solder paste from the openings, and because they are metal, they are not as easily worn as rubber scrapers, so they do not need to be sharp. However, they are much more expensive than rubber scrapers and may cause template wear.
The use of different squeegee types is distinguished in printed circuit assembly (PCA) using standard components and dense-foot components. The requirements for the amount of solder paste vary greatly for each component. Fine-pitch components require much less solder volume than standard surface mount components. The pad area and thickness control the amount of solder paste.
Some engineers use double-thickness templates to apply the proper amount of solder paste to dense-footed components and standard surface mount pads. Other engineers take a different approach, using a more economical metal scraper that doesn’t need to be sharpened as often. It is easier to prevent changes in the amount of solder paste deposition with a metal squeegee, but this method requires an improved template opening design to prevent excessive solder paste deposition on the fine-pitch pads. This method has become more popular in industry, but the use of double-thickness printing with rubber squeegees has not disappeared.
SMT Stencil type
Important printing quality variables include the accuracy and finish of the steel mesh wall. It is important to preserve the proper aspect ratio of the width and thickness of the steel mesh. The recommended aspect ratio is 1.5. This is important to prevent template blocking. Generally, if the aspect ratio is less than 1.5, the solder paste will remain in the opening. In addition to the aspect ratio, as recommended by IPC-7525 “Steel Mesh Design Guide”, there must be an area ratio greater than 0.66 (the area of the land divided by the area of the hole wall). IPC-7525 can be a good start for steel mesh design.
The process of making the hole controls the smoothness and precision of the hole wall. There are three common processes for making stencils: chemical etching, laser cutting, and additive processes.
Solder paste printing output
In a given time period, how many qualified printed circuit boards can be produced? There are many factors that affect the output of an SMT production line. One factor that is often mentioned is the cycle of solder paste printing equipment. In the past, “machine cycle” was used as an important indicator of the production output of major equipment, but for a stencil printing machine or other electronic manufacturing equipment, it was only a factor in measuring the true output. The electronics manufacturing industry often interchanges the terms “service cycle” and “output”. In fact, they are two different factors in the measurement of machine performance.
Cycle
The definition of cycle is the speed of the basic functional tasks, such as loading and unloading of circuit boards, that the machine can complete. This generally includes the following steps: the movement of the circuit board in and out of the machine, the circuit board being calibrated according to the set target (template reference mark), the circuit board moving to its necessary position, and the circuit board being transferred to the next process. The actual completion of the main function of the machine (in this case, the actual printing of solder paste) generally depends on the recognized elements that define the machine cycle. In most cases, the supplier of solder paste printing equipment only defines the cycle of the machine as the process in which the printed circuit board is fed into and out of the machine, and the printed circuit board is calibrated according to the set target (template reference mark).
Is a squeegee or closed print head used?
A closed print head saves the time spent applying solder paste to the template. Even if an automatic solder paste application system is used, the machine takes time to apply new solder paste to the template. When switching from one type of circuit board to another, the closed print head shows a unique advantage, because all the solder paste has been installed in the closed print head. Before cleaning the template, only a small amount of solder paste is scraped off the template. Since the solder paste of the next product is already installed in the print head, the waste of solder paste is also very small.
Solder paste coating: How to apply solder paste to the template when using a squeegee. The factors that affect it include the coating method (manual or automatic coating), as well as the opening density and the size of the PCB, which will determine the frequency of solder paste replenishment.
“Ease of use” of operating software
The software must be easy to use. The operation of all controllable functions must be easy to understand. The software interface must be as intuitive as possible to simplify operation. This facilitates the assembly, conversion and normal operation of the machine, and has a great influence on the long-term production output of the system.
The frequency and method of template cleaning
All solder paste printing processes require cleaning templates at a certain frequency. The frequency of stencil wiping is a function of many variables, including stencil design, final surface treatment of the printed circuit board (hot air leveling HASL, silver immersion, nickel/gold immersion, organic solderability protective layer OSP, etc.), printing process In the circuit board support, etc. Even the most optimally designed solder paste printing process must include stencil cleaning, so we must evaluate how a certain machine accomplishes this function. All modern solder paste printing equipment provides a template cleaning function. It must be clear whether it is necessary to use a vacuum or solvent to assist the cleaning work when performing the template cleaning function.
Separation distance and speed from the template to the circuit board must also be known. All systems are different. Due to the increasing density, some PCB boards require a slower separation speed to improve the separation of the template and the deposited solder paste.
After solder paste printing inspection
Most modern solder paste printing equipment provides two-dimensional (2D) post-press inspection functions, and some can also provide three-dimensional (3D) post-press inspection functions for solder paste deposition in key equipment. All 2D and 3D post-press inspection systems work differently. Therefore, it is important to understand the various variables and methods that can be measured, and how to use the result data, which is very important for evaluating the value of additional work.
Assembly and conversion plan, including related MTTA
When changing from one product to another, a lot of solder paste printing equipment conversion work is required. Many solder paste printing processes require several conversions in a day. It must be clear how long it will take your equipment to switch from one product to another. Which product conversion variables are particularly important for the optimal operation of the machine?
Process Statistical Control Strategy (SPC)
As mentioned above, throughput is the number of qualified circuit boards assembled in a given time period. Process quality is critical to achieving the highest throughput, so it is necessary to understand the process operation as “real-time” as possible. We cannot focus on remedial optimization work through the defects found after the end of the production run. We must promote a “forward-looking” type of production to prevent the formation of a “reactive” production system that is only used to find defects.
Solder paste removal method
Question: Can a small spatula be used to remove misprinted solder paste from the board? Will this get solder paste and small solder balls into holes and small gaps?
Answer: Using a small spatula to remove the solder paste from a misprinted board may cause some problems. It is generally feasible to immerse the misprinted board in a compatible solvent, such as water with some additives, and then use a soft brush to remove the small tin beads from the board. I would rather soak and scrub repeatedly than violently dry-brush or scrape. After the solder paste is printed, the longer the operator waits to clean the misprint, the more difficult it is to remove the solder paste. Misprinted boards should be placed in the soaking solvent immediately after the problem is discovered, because the solder paste is easy to remove before it dries.
Avoid wiping with cloth strips to prevent solder paste and other contaminants from smearing on the surface of the board. After soaking, scouring with a gentle spray can often help remove unwanted solder paste. It is also recommended to use hot air for drying. If a horizontal stencil cleaner is used, the side to be cleaned should face down to allow the solder paste to fall off the board.
As usual, paying attention to some details can eliminate undesirable situations, such as misprinting of solder paste and removal of cured solder paste from the board. It is our goal to deposit an appropriate amount of solder paste at the desired location. Dirty tools, dried solder paste, and misalignment between the template and the board may cause undesirable solder paste on the bottom surface of the template or even the assembly. During the printing process, the template is wiped according to a certain rule between printing cycles. Ensure that the template is located on the pad, not on the solder mask, to ensure a clean solder paste printing process. Online, real-time solder paste inspection, and inspection before reflow after component placement, are both process steps that help reduce process defects before soldering occurs.
For fine-pitch templates, if the thin template cross-section bending causes damage between the pins, it will cause solder paste to deposit between the pins, causing printing defects and/or short circuits. Low-viscosity solder paste may also cause printing defects. For example, the high operating temperature of the printer or the high squeegee speed can reduce the viscosity of the solder paste in use, and cause printing defects and bridging due to the deposition of excessive solder paste.
In general, a lack of adequate control over materials, solder paste deposition methods and equipment are the main causes of defects in the reflow soldering process.
Matters for using solder paste
Stir
- Manual stirring: Take the solder paste out of the refrigerator, open the lid after returning to room temperature (at 25°C, it takes about three to four hours), and stir the solder paste thoroughly with a stirring knife. If the cap is broken, the solder paste will become a tin mass due to moisture absorption.
- Using an automatic mixer: If the solder paste is only briefly warmed up after being taken out of the refrigerator, an automatic mixer is needed. Using automatic mixing will not affect the characteristics of the solder paste. After a period of stirring, the solder paste will gradually warm up. If the mixing time is too long, it may cause the solder paste to be warmer than the operating room temperature, causing the solder paste to be poured on the board in a whole, and causing bleeding during printing, so be careful. Due to different machines, changes in room temperature and other conditions, different mixing times will be required, so please perform enough tests before proceeding.
Printing conditions
Scraper metal products or urethane products (hardness 80-90 degrees)
Scraper angle 50-70 degrees
Scraper speed 20-80㎜/s
Printing pressure 10-200 KPA
Installation time
Complete the installation of the parts within six hours after applying the solder paste. If left for too long, the solder paste will harden and the parts will be inserted incorrectly.
Precautions
- Individual physiological reactions to solder paste will vary. In order to be cautious, try to avoid inhaling the fumes released by the solvent during the operation, and avoid extended contact with the skin and mucous membrane tissue.
- The solder paste contains organic solvents.
- If the solder paste gets on the skin, wipe it off with alcohol and rinse thoroughly with water.
