Soldering paste is a critical component in the process of surface mount device assembly. It fills tiny gaps between components and provides a conductive medium for heat to flow from the soldering iron, through the solder joint, and into the metal on either side of it. This first blog post will explore what solder paste is and how to use it properly during SMD assembly!
What Is a Solder Paste?
A solder paste is a mixture of metal alloys, fluxes, and solvents that are created into a thick sticky paste.
This thin layer serves as the medium for attaching components to printed circuit boards (PCBs).
Solder pastes come in different thicknesses from very thin down to approximately 50 microns. The thicker formulations can be used with stencil printing whereas thinner ones require screen-printing techniques. Thicker types will stay on the PCB surface better than thinner ones but it also depends on other factors such as print technique and curing conditions [1].
Solder Paste Grades:
Rosin-Based Solder Pastes
Rosin-based solders typically have better elongation, which means they stretch more before breaking apart. This type has higher print speeds compared to water/alcohol-soluble ones because it requires less dwell time on each component pad after printing. They also require low curing temperatures. However, this could cause issues like delamination during rework because there might not be enough heat left to melt the adhesive materials in the solder joint.
Water Soluble Flux-Based Solder Pastes
Water-soluble fluxes are typically used with leaded solder alloys. These water-soluble pastes do not require high baking temperatures to activate the flux, so they can be cured at a lower temperature than rosin-based ones that means less degradation in component performance. It also allows for a short development time so the components can be placed and soldered in one step, if needed. This type may cause corrosion issues with lead-free alloys because there is no residue to protect parts from oxygen exposure.
No-Clean Solder Paste
What Are the Advantages of Solder Paste?
Solder paste is the easiest way to apply solder for SMD components. It’s also more accurate than using a stencil and hand-placing, so it requires less work on your part. You can also use solder paste for anything from prototyping to full production.
The following are a few of the benefits of using solder paste:
- Accurate solder placement;
- No messy brushes needed;
- Easy board cleanup and re-work ability;
- High yield soldering results in less time spent on manufacturing boards;
What are the disadvantages of solder paste:
- Solder paste is harder to clean up;
- It takes longer than stenciling and hand placement;
- Requires more stringent process control procedures (requires a lot of testing);
How to Use Solder Paste:
1) Using Stencils and Spatulas:
- Stencils are usually made of stainless steel, copper, or polyimide. There should be one stencil for each side of the PCB so that solder paste can be applied to both sides;
- To apply solder paste you need a special squeegee called “spatula” which is between 0.75mm and 12mm thick. This tool must not stick to any surface, otherwise, it will mess up your work if pulled out with the clogged solder paste on its edge! Spatulas also come in different shapes like straight-line ones and triangle ones depending upon their usage area (e.g., limited working space areas);
2) Using Jet Printer
Solder paste can be applied using the “jet printing” method [2] which is also known as screen printing.
It is relatively easy but the only drawback here is that you cannot apply a small quantity of solder at one go, rather multiple passes must be made for each side while adding more solders after every refill cycle:
- If your printer doesn’t come equipped with an automatic feed mechanism then a manual feeding system should do fine;
- Print heads are available in various shapes and sizes according to the requirements of different PCBs;
- A flat print head with several nozzles for solder paste deposition works best on large boards but needs a lot of maintenance;
While single slot or multiple nozzle print heads can be used successfully even if their number is less than four, because they lack precision when depositing solder paste onto small pads. However, these are cheaper options compared to ones having more nozzles.
Assembling the Electronic Components
Assemble all of the components that will go into our solder paste stencil over some kind of protective material such as wax paper or aluminum foil so that they don’t stick when you do your first printing pass with the solder paste:
- Small electronic components (resistors, capacitors);
- Mounted SMD chips (chips already mounted onto carrier boards like breakout boards);
- Circuit Boards (breakout boards, custom PCBs);
Here is what you should do to assemble electronic components with solder paste:
- You need to put down a layer of solder paste on the stencil and then place all our components in their correct location. Use tweezers for this step;
- Put some solder paste onto the stencil using either an eyedropper or syringe that can be bought at any hobbyist store. Try to only use enough so that it’s just covering up most of each hole leaving room for error later when you’re placing your parts. For bigger chips with more pins, it’s good practice to have extra space around them as well – but not too much that they fall out! One component into every single hole, carefully and neatly;
- Take a piece of glass like a microscope coverslip (for example) and place it over the stencil with the components;
- Press down hard all around each component to ensure that it sticks well onto the solder paste underneath;
- Do this for every single one of your components! When you’re done, let them sit there overnight so that they can properly bind to our solder paste – making sure not to move anything during this step, or else parts could fall off later when we try printing out our circuit board design!
It’s best if you have multiple sets of tweezers for this part since some users tend to touch their PCBs while placing them on these tiny little electronic pieces!
Solder Paste Storage
The first thing you need to understand about solder paste is that it has a shelf life, just like many food items or other products on your grocery list. This means if not stored correctly, it can spoil! You may notice some changes in color and consistency when this happens.
Experts recommend storing solder pastes between 66˚F and 77˚F [3]. If your facility experiences higher ambient temperatures during the summer months, consider storing one paste at a lower temperature.
The most common method to store solder paste is in an air-conditioned room or cooler, although there are some other methods including storing the product on refrigerated shelving units and using nitrogen blanket systems. In many cases, it’s also possible to use existing storage racks but needs to be checked against local fire codes for safety measures during hot months of operation.
As long as solder pastes have been processed properly, they should withstand elevated temperatures without performance degradation. It’s important that you check with your supplier if you are unsure about how their particular products will react under certain conditions since different formulations exist within the industry.
Comparison of Different Types of Solder Paste: Alloy Composition, Melting Point, Flux Type, Viscosity, and Particle Size
In this table, we compare different types of solder paste based on their alloy composition, melting point, flux type, viscosity, and particle size to help you choose the right solder paste for your application.
| Type of Solder Paste | Alloy Composition | Melting Point | Flux Type | Viscosity | Particle Size |
|---|---|---|---|---|---|
| Sn63/Pb37 | 63% tin, 37% lead | 183°C (361°F) | Rosin-based | 120,000 cps | 20-45 µm |
| Sn96.5/Ag3.0/Cu0.5 | 96.5% tin, 3% silver, 0.5% copper | 217°C (423°F) | No-clean | 500,000 cps | 5-25 µm |
| Sn99/Ag0.3/Cu0.7 | 99% tin, 0.3% silver, 0.7% copper | 227°C (441°F) | Water-soluble | 150,000 cps | 5-20 µm |
| Sn42/Bi58 | 42% tin, 58% bismuth | 138°C (280°F) | Rosin-based | 30,000 cps | 10-25 µm |
The table includes four different types of solder paste, along with information about their alloy composition, melting point, flux type, viscosity, and particle size. This information can be useful for choosing the right type of solder paste for a particular application, based on factors such as the desired melting point, ease of use, and compatibility with other materials.
Learn more guides to improve your knowledge in soldering:
FAQ
1. What is solder paste composition?
Soldering paste is a mixture of metal powder and flux. The metal powders can be made from different materials such as Sn, Pb, and alloys like SAC or lead-free solder alloy.
The major components in the solder pastes are [4]:
- Metal Powders (Sn/Pb, lead-free);
- Flux system (water-soluble vs. non-water soluble);
- Thickening agent(s), carriers & additives;
2. What does a solder paste do?
Solder paste is applied to the pads on a printed circuit board (PCB) using an ink-jet or stencil printing process. It allows for controlled placement of solder onto components during assembly, which provides excellent electrical conductivity between components and PCBs through joints called fillets.
3. How do you use solder paste by hand?
You can use a squeegee, blade, or finger to spread solder paste. The flat side of the tool should be used on larger boards and stencils to ensure an even film thickness over all pads.
For small stencil sizes (less than 50×50 mm), you may find it easier using pressure from just one finger as opposed to a whole hand for applying the paste.
Many times those who are new to solders paste application will start with too thick of a layer which makes spreading difficult. In these cases, try pressing harder but also move back and forth in order to evenly distribute the material across the target surface. It’s best not to go overboard since this method might cause shorting between adjacent contacts due to excess solder build-up.
You can also use tweezers with a non-stick tip to pick up small components and place them on the board before heating it up, however, you don’t want to contaminate solder paste with fingerprints or other types of dirt during this process. When using tweezers for placing tiny parts, try wearing gloves if possible which will help prevent unwanted oils from your fingers from getting into contact with nickel-plated surfaces.
4. How do you use solder paste without a stencil?
If you don’t have access to an adhesive stencil, there are other methods for applying solder paste. One option is the “print and place” method which entails printing out a solder paste image onto paper using an ink-jet printer after coating it with flux.
5. What is the difference between solder flux and solder paste?
The solder is used to join components on your circuit board, while the flux is used before assembly to clean and prepare your board for soldering.
Because the metal on your printed circuit board is unprotected, it may become contaminated with impurities from handling and oxide compounds formed due to the metal’s exposure to air. Soldering can be hampered by foreign particles of sand, which prevent the connection between the circuit board metal and components’ leads from forming solid joints.
Prior to soldering, it is critical that the board be cleaned with flux in order to eliminate this contamination. Solder flux not only removes these contaminants but also aids in the wetting of the molten solder between the metals and prevents re-oxidation of the surfaces during soldering [5].
6. What is solder paste for jewelry?
Gold solders are mainly used to make gold jewelry, whereas copper, brass, and bronze solders are mostly used to make copper, brass, and bronze jewelry.
7. Does solder paste dry?
Solder pastes are water-soluble, which means that they require moisture to chemically react with the metals on your printed circuit board. If you don’t have access to humidity in order for solder paste to be activated, it defeats its original purpose of joining components together due to improper activation of flux.
It’s important not to let solder paste dry out because doing so will cause it to become unusable and potentially damage the materials surrounding it when heating them up. You should always store solders in an airtight container or plastic bag after opening.
Additionally, keep in mind that some lead-free alloys may need extra time exposed under heat since their melting points can take longer than traditional Sn/Pb alloy solder paste formulations.
8. Is soldering paste conductive?
It is critical to keep circuits from faulting and to maintain the machine at an optimal production level as long as possible, which is where solder flux comes in. Solder flux aids in the prevention of oxidation during the soldering process while also improving electrical contact.
When soldering, the use of any type of flux can make it conductive. Because conductivity degrades the performance of your device, you should clean it after applying it [6].
Useful Video: Pro Tips: What is Solder Paste?
References:
- https://assets.testequity.com/te1/Documents/ARTICLE_LIBRARY/Indium%20-%20Soldering%20101.pdf
- https://www.techbriefs.com/component/content/article/tb/techbriefs/manufacturing-prototyping/23681
- https://aimsolder.com/sites/default/files/aim_paste_handling_guideline_revnf1.pdf
- http://www.electronicsandyou.com/blog/solder-paste-and-its-application-in-smt.html
- https://www.tempoautomation.com/blog/understanding-soldering-part-2-the-difference-between-flux-and-solder/
- https://weldingmastermind.com/is-solder-flux-conductive-heres-the-truth/