How to Use Solder Paste: Stencil, Syringe, and Soldering Iron Methods

Key Takeaways: How to Use Solder Paste

● Solder paste must be applied before component placement and reflowed with a controlled heat profile.

● Most SMT defects are caused by incorrect paste volume, poor storage, or improper heating.

● Stencil printing provides the most consistent and reliable results.

● Manual methods (syringe or iron) are suitable only for low-density or repair work.

Solder paste is the foundation of modern electronics assembly. In simple terms, it is a mixture of microscopic solder spheres and flux held together in a creamy consistency. Unlike traditional wire solder, it is applied before components are placed, acting as a temporary adhesive before melting to form permanent electrical joints.

Correct usage is extremely important since a majority of SMT defects, like bridges, cold joints, and shifting of components, are caused by either incorrect paste volume or bad handling. JLCPCB uses automated jet printers and 3D SPI (Solder Paste Inspection) to guarantee perfection.

Applying solder paste on the stencil

Learn More: The Role of Solder Paste vs Flux

Before You Start: How to Choose and Prepare Solder Paste

Before you squeeze the syringe, ensure you have the correct material and that it is ready for use.

How to Choose the Right Solder Paste

Not all pastes are the same. For general SMT work, SAC305 (Sn96.5/Ag3.0/Cu0.5) is the industry standard for lead-free assembly. If you are doing repair work where lower temperatures are needed, leaded paste (Sn63/Pb37) is easier to use but carries health risks.

You also need to choose the right Particle Size (Type).

● Type 3: Standard for most components.

● Type 4: Finer powder, essential for small components (0402 and smaller).

Note: JLCPCB's SMT assembly typically uses Type 4 lead-free solder paste optimized for fine-pitch components, ensuring consistent stencil release and minimal bridging.

Note: For hand soldering, "No-Clean" flux is highly recommended so you don't have to wash the board aggressively after reflow.

How to Store and Thaw Solder Paste Correctly

Solder paste is perishable. If you don't treat it right, it will dry out or spatter.

1. Store Cold: Keep paste in a fridge at 2–10°C. This stops the flux from reacting with the metal.

2. Thaw Naturally: Remove the paste from the fridge 3–4 hours before use. Do not open the jar/syringe while cold. If you open it cold, moisture from the air condenses on the paste, causing "solder balls" during heating.

3. Mix Gently: In case of using a jar, gently mix for 1 minute so that the flux is uniformly spread.

4. Working Time: After applying the paste on the stencil, it can be worked for approximately 4–8 hours until it dries out.

Note: Humidity and temperature can reduce this to 2–3 hours.

Essential Tools Needed for Using Solder Paste

● Solder Paste: (Syringe or Jar).

● Stencil: Laser-cut stainless steel stencils are best. JLCPCB offers these with "Electropolishing" to smooth the walls for better paste release.

● Squeegee: A metal blade or even an old credit card.

● Tweezers: Fine-tip for placing components.

● Heat Source: Hot air gun, reflow hot plate, or a specialized reflow oven.

Note: There are three main ways to apply solder paste, depending on your tools and the complexity of your printed circuit boards.

Method 1: How to Use Solder Paste With a Stencil (Best Practice)

This is the most reliable method and mimics the professional process used at JLCPCB. A personalised stencil makes sure that every pad receives the precise mathematical volume of paste that is needed.

SMT stencil printing process, with the angles of the squeegee and solder paste moving into the apertures.

Using Solder Paste With a Stencil: A Step-by-Step Guide

1. Align & Secure: Tape your PCB to a flat surface. Place scrap PCBs around it to create a flat "surround." Tape the stencil over the PCB, aligning the holes exactly with the pads.

2. Apply Paste: Squeeze a bead of paste onto the stencil (above the pattern, not on it).

3. The Sweep: Hold your scraper/squeegee at a 45° angle. Drag the paste across the pattern with firm, consistent pressure. You want to fill the holes, not leave a thick layer on top of the stencil.

4. Lift: Lift the stencil straight up vertically. Do not slide it.

5. Inspect: The paste deposits should look like little clean bricks on the pads.

6. Place & Reflow: Position the components and then apply heat right away.  

Common Mistakes:

● Too much pressure: Soldering material scoops out from the holes (lack of solder).

● Not enough pressure: Leaves a thin layer of paste on top of the stencil (short circuit).

● Misalignment: When the paste touches the solder mask, it could end up as solder balls.

Method 2: How to Use Solder Paste With a Syringe

Best for repairs or one-off prototypes where ordering a stencil isn't feasible.

Applying Solder Paste With a Syringe: A Step-by-Step Guide

1. Needle Selection: Choose either a pink (#20) or blue (#22) tapered tip.

2. Dispense: Keep the syringe in an upright position. Apply a little pressure to come out with a minute dot on every pad.

3. Volume Control: Try to cover just 50-60% of the pad. When it melts, the paste will spread.

4. Place: Drop the component on top. Do not push down hard.

5. Reflow: Heat with hot air.

Practical Tip: Do not use a syringe for QFN or BGA packages. It is impossible to get the volume consistent by hand, leading to hidden shorts. Always use a stencil for these parts.

Use Solder Paste With a Syringe

Method 3: How to Use Solder Paste With a Soldering Iron

This is a specialized method that can be utilized for making fast repairs or for temporarily fixing big parts.

How to do it:

1. Apply a tiny amount of paste to the pad (and component lead if possible).

2. Preheat the pad slightly by holding the iron tip near it.

3. Touch the pad (not the paste directly) with the iron's tip. The heat will flow, making the flux work and melting the paste into the joint.

Warnings:

● Splatter: If a hot iron (350°C+) touches wet paste directly, the solvents can boil instantly, and this will result in the hot solder being splattered.

● Flux Burn: Overheating kills the flux before it can clean the oxide, resulting in a gritty joint.

How to Reflow Solder Paste After Application

Applying the paste is only half the battle. You must heat it correctly (Reflow) to create a solid electrical bond. Professional assembly houses like JLCPCB use 10-zone nitrogen ovens to control this profile perfectly, but you can mimic it at home.

Learn More: Reflow Soldering Principles and Precautions

The solder paste reflow profile with temperature zones: preheat, soak, reflow, and cooling.

#1: Using a Hot Air Gun

● Set air flow to medium (so you don't blow components away).

● Preheat the whole board area to approximately 150°C for half a minute.

● Focus the heat on the component, moving the nozzle in circles. Keep the hot air nozzle 3–5 cm away to avoid blowing components off the pads or overheating them.

● Visual Cue: Look for the gray paste to change to silver and shiny. This is the "wetting" action. Remove heat once all joints are shiny.

#2: Using a Reflow Plate or Mini Oven

● Place the PCB on the cold plate/oven.

● Set temperature to ~230°C (for Sn63) or ~250°C (for SAC305).

● Allow it to ramp up. You will see the flux bubble first, then the solder will melt.

● After the solder has melted, switch off the heat and allow cooling on its own. Do not blow on it because quick cooling could destroy ceramic capacitors.

How to Tell if Solder Paste Reflow Is Successful (Visual Inspection Checklist)

● Self-Alignment: As the solder melts, surface tension naturally pulls the component into the center of the pad. If you see the part wiggle slightly into place, it is a sign of good wetting.

● Good Joints: Joints are smooth, shiny, and concave (fillet shape).

● Poor Joints: Joints appear dull, grainy, or ball-shaped (non-wetting or cold joint).

Troubleshooting: How to Fix Common Solder Paste Defects

Engineers, regardless of experience, encounter problems. Here's a guide to rapid solutions.

Common SMT solder paste defects: bridging, tombstoning, and solder balls.

How to Handle and Clean Up Solder Paste Safely

Solder paste contains chemicals and metals that require care.

● Clean Immediately: Uncured paste is easy to wipe off with Isopropyl Alcohol (IPA). Cured paste is very hard to remove.

● Don't Reuse: Once paste has been on the stencil, do not scoop it back into the fresh jar. Store it in a "waste" jar.

● Safety: The flux can be a skin irritant, and lead (if using leaded paste) is toxic. Always wear gloves and wash your hands thoroughly. Use a fume extractor when reflowing.

When You Should NOT Use Solder Paste Manually

Although applying manually is suitable for 0805 resistors or SOIC packages, it has its limits. You should consider professional JLCPCB SMT Assembly if your design includes:

● Fine-Pitch ICs: For pitch sizes less than 0,5 mm, it is practically impossible to stencil manually without creating a short circuit between the pads.

● QFN / Thermal Pads: It is hard to control the paste coverage on large ground pads manually, leading to parts floating or tilting.

● BGA Parts: These connections need to be verified through X-ray examination.

The Professional Alternative: JLCPCB provides low-cost SMT services where we take care of everything:

1. Automated Printing: Precise electropolished stencils.

2. Type 4 Paste: Standard for high-precision boards.

3. 3D SPI: We inspect every single pad for volume and offset.

4. Reflow Ovens: JLCPCB employs contemporary, multi-zone reflow ovens built for uniform thermal control.

Visual comparison of a good shiny solder fillet vs a bad cold solder joint.

Conclusion

Gaining the skill to apply solder paste reveals a whole new area of electronic design, which permits the use of smaller, more powerful parts. The method depends on your equipment: a stencil for perfect quality, a syringe for rapid prototyping, or a soldering iron for repairs.

However, the final quality always depends on the heating profile and precise application volume. For complex, high-density boards, save yourself the headache of troubleshooting bridges and cold joints by utilizing JLCPCB’s PCBA Service.

FAQs about Solder Paste

Q1.  What is the appropriate amount of solder paste to use?

In the case of manual syringe application, a dot covering around half the pad area is the target. The height is to be approximately 0.1mm. For stencil usage, the thickness of the stencil sets the volume automatically.

Q2. Can I reflow solder paste twice?

Technically, yes, but it is risky. Re-heating increases the intermetallic layer thickness, which can make the joint brittle. It also exhausts the flux. If reworking, always add fresh flux.

Q3. What is the best temperature for solder paste?

For Leaded (Sn63/Pb37) paste, the peak temperature should be ~210–220°C. For Lead-free (SAC305) paste, the peak should reach ~235–245°C.

Q4. How long can solder paste stay on the PCB before reflow?

The best-case scenario would be the formation getting done within two hours of the paste being applied. In the case of a longer time, the flux solvents will evaporate, and the paste will become "tack dry," which will then cause poor wetting and component adhesion problems.

Q5. Is solder paste or wire solder better for SMD?

Solder paste is far superior for SMD assembly because it allows you to place components flat on the board and reflow them all at once. Wire solder requires holding the part with tweezers while manipulating the iron and wire, which is difficult for tiny parts.