Solder Paste: Types, Alloys, Mesh Sizes & How to Choose

Solder paste is a mixture of powdered solder alloy suspended in flux that you print onto PCB pads to bond surface-mount components during reflow. Two variables decide whether it prints, melts, and holds: the alloy, which sets the melting point and joint strength, and the powder particle size (mesh), which sets how fine a feature you can print. Pick the wrong combination and you get bridging on fine-pitch parts, voids under the BGAs, or starved joints that pass inspection and fail in the field. This guide covers the main solder paste types — by alloy, by flux, and by powder mesh — with the IPC numbers you actually need and a decision framework for choosing the right paste for your build.

Solder Paste at a Glance: Key Takeaways

• Solder paste is roughly 88–91% solder alloy powder by weight plus flux; a printed deposit loses about half its volume at reflow as flux burns off.
• Alloy sets the melting point: SAC305 (lead-free) reflows around 217–220°C, eutectic Sn63/Pb37 melts at 183°C, and low-temp Sn/Bi melts near 138°C.
• Flux type (no-clean, water-soluble, RMA) decides cleaning and reliability and is classified under IPC J-STD-004.
• Powder mesh (Type 3–6 under J-STD-005) sets the finest pitch you can print — and finer powder is not automatically better.
• Match powder to your smallest stencil aperture with the 5-ball rule, keep apertures at or above a 0.66 area ratio, and refrigerate paste at 2–10°C.

What Is Solder Paste? Composition and How It Works

Solder paste is a homogeneous suspension of spherical solder alloy powder in a flux vehicle. By weight it is about 88–91% metal; the rest is flux — a blend of resin or rosin, solvent, activators, and rheological additives that control how it prints and slumps. Here is the number that trips up first-timers: although paste is roughly 90% metal by weight, only about half of a printed deposit’s volume ends up as solid solder. The flux and carrier outgas during reflow, so a tall, crisp paste brick collapses into a much smaller joint. That gap is why aperture and volume math matters — what you print is not what you get.

The paste does three jobs in sequence. Before reflow, the tacky flux holds placed components in position through board handling and transport, acting as a temporary adhesive. During reflow, the flux strips oxides off the powder, the pads, and the leads, and shields them from re-oxidation while the alloy is molten so the solder can wet and coalesce. After the alloy melts above its liquidus and cools, it forms the permanent electrical and mechanical joint and grows an intermetallic layer at the interface — for SAC alloys on copper that is Cu6Sn5 — which is what actually bonds the joint metallurgically.

That sequence is why paste is the single highest-leverage material in surface-mount (SMT) assembly. Defect studies on production lines repeatedly trace the majority of failures back to the paste and the print step, not to placement or the oven. Get the paste and the print right and most of the rest of the line behaves.

Solder Paste Types by Alloy: SAC305, Leaded, and Low-Temp

The alloy is the first decision because it sets the melting point, the reflow profile your oven has to hit, and how the finished joint behaves mechanically. The common families:

Alloy	Composition	Melting Point	Best For
SAC305	Sn 96.5 / Ag 3.0 / Cu 0.5	217–220°C	Default lead-free SMT; RoHS markets
SAC387	Sn 95.5 / Ag 3.8 / Cu 0.7	217–219°C	Slightly better wetting; near-eutectic
SAC105	Sn 99.0 / Ag 1.0 / Cu 0.5	~225°C	Drop/shock resistance; handhelds, wearables
Sn63/Pb37	Sn 63 / Pb 37 (eutectic)	183°C	RoHS-exempt high-rel; prototyping; rework
Sn62/Pb36/Ag2	Sn 62 / Pb 36 / Ag 2	~179°C	Leaded + silver; silver-bearing pads
Sn42/Bi58	Sn 42 / Bi 58 (eutectic)	138°C	Low-temp, heat-sensitive parts, LEDs
Sn/Cu (Sn0.7Cu)	Sn 99.3 / Cu 0.7	227°C	Lower-cost lead-free; wave/selective

SAC305: The Lead-Free Workhorse

SAC305 is the default lead-free alloy for most SMT work: 96.5% tin, 3.0% silver, 0.5% copper, with a solidus of 217°C and a liquidus of 220°C. It is RoHS-compliant, wets well, forms strong joints, and is supported by every paste maker and reflow oven on the market — which is most of why it won. The cost is heat. That 217–220°C melt means peak reflow temperatures around 240–250°C, roughly 30–40°C hotter than a leaded process, which stresses heat-sensitive parts and warps thin or large packages more. The silver content also makes it pricier than leaded paste, which pushes some buyers toward lower-silver variants: SAC387 trades a little cost for marginally better wetting, while SAC105 drops silver to improve drop-shock resistance for handheld and wearable products at the expense of a slightly higher melting point.

Leaded Sn63/Pb37: Still the Easiest to Process

Eutectic tin-lead — 63% tin, 37% lead — melts sharply at 183°C with no plastic range, which makes it forgiving. It wets aggressively, reworks cleanly, and produces shiny, low-void joints with a wide process window. RoHS pushed it out of most consumer electronics, but it is still specified under RoHS exemptions for aerospace, defense, medical, and industrial hardware, where decades of field data and low thermal stress matter more than lead-free compliance. If you are prototyping or doing hand rework, leaded paste is simply easier to get right.

Low-Temperature Sn/Bi: Gentle on Heat-Sensitive Boards

Tin-bismuth pastes — typically Sn42/Bi58 (eutectic at 138°C) or Sn42/Bi57/Ag1 — reflow around 138–170°C, roughly 80°C cooler than SAC305. That low peak protects temperature-sensitive components, large warp-prone boards, and LED packages that degrade above 200°C, and it cuts oven energy. The trade-off is mechanical: plain Sn/Bi joints are brittle and fail under drop and bend stress, so they are a poor fit for handhelds unless the bismuth is modified or the design is mechanically supported. Low-temp paste is a targeted tool, not a general-purpose replacement for SAC305.

Lead-Free Solder Paste and RoHS Compliance

Lead-free solder paste became the default after the EU RoHS directive restricted lead in most electronics from 2006, with REACH and regional equivalents reinforcing it. For anything sold into the EU and most global consumer markets, lead-free — usually SAC305 — is mandatory. Three practical consequences follow.

First, your reflow profile changes. Lead-free liquidus temperatures sit 30–40°C above leaded, so the soak and peak zones move up and the process window narrows; running SAC305 on a leaded profile is a classic cause of cold joints and poor wetting. Second, tin-rich lead-free alloys can grow tin whiskers — thin conductive filaments that bridge fine-pitch features over time — which matters for high-density and high-reliability designs and is managed with alloy choice, conformal coating, and standoff. Third, lead-free joints tend to void more than leaded, so paste selection and profile optimization carry more weight on power devices and large thermal pads.

Leaded paste has not disappeared. RoHS carves out exemptions for defense, aerospace, certain medical and industrial categories, and server-class hardware, where the reliability record and lower processing stress of Sn63/Pb37 still win. Know which regime your product falls under before you spec the paste — it constrains everything downstream.

Solder Paste Types by Flux: No-Clean, Water-Soluble, and RMA

The flux carrier is classified by IPC standard J-STD-004 and decides two things you will live with: how active the chemistry is (how well it wets oxidized surfaces) and whether you have to wash the board afterward.

Flux Type	Cleaning	Activity & Wetting	Best For
No-clean	None required	Mild; inert residue left on board	Consumer and most commercial boards; longest shelf life
Water-soluble (OA)	Mandatory DI-water wash	Aggressive; superb wetting on oxidized surfaces	High-reliability or zero-residue builds
RMA (rosin)	Solvent wash when needed	Low to moderate; rosin-based residue	Established high-rel work where rosin residue is accepted

One myth worth killing: “no-clean” does not mean “always safe to leave.” No-clean residue is engineered to be inert under typical conditions, but it can still interfere with conformal-coating adhesion, sit on in-circuit test (ICT) points and block probe contact, or cause leakage across high-impedance and RF nodes. On those boards you either clean anyway or qualify the residue against your end-use. Water-soluble flux is the opposite trade: it wets heavily oxidized surfaces beautifully, but its residue is corrosive and must be fully removed with a deionized-water wash, or it will degrade the joint over time. RMA sits between them and survives mostly in high-reliability work where rosin residue is understood and acceptable.

Solder Paste Mesh Sizes and Particle Types (J-STD-005)

Solder paste powder is sorted by particle size into numbered “types” under J-STD-005, currently revision J-STD-005B (January 2024). The rule is counterintuitive at first: the smaller the type number, the larger the particles. At least 80% of the powder by count must fall inside the listed size band for a paste to carry that type label.

Powder Type	Particle Size	Mesh	Typical Components / Pitch
Type 1	75–150 µm	—	Coarse; rarely used in modern SMT
Type 2	45–75 µm	—	Larger components; mostly legacy
Type 3	25–45 µm	-325/+500	Standard SMT, ≥0.5 mm pitch (0603, 0805)
Type 4	20–38 µm	-400/+635	Fine pitch, 0201, 0.4 mm pitch — today’s mainstream
Type 5	15–25 µm	—	Ultra-fine: 01005, micro-BGA, ≤0.3 mm
Type 6	5–15 µm	—	microLED, CSP, SiP, advanced packaging
Type 7	2–11 µm	—	Specialized ultra-fine / jetting

For years Type 3 was the SMT default, and it still handles standard work down to about 0.5 mm pitch — 0402 and 0603 chips, ordinary QFPs. As 0201 and 0.4 mm-pitch parts took over, Type 4 became the mainstream choice and now dominates most assembly lines; its 20–38 µm powder releases cleanly through smaller apertures. Type 5 reaches 01005 components and micro-BGAs, and Type 6 pushes into microLED, chip-scale package (CSP), and advanced-packaging territory.

The temptation is to spec the finest powder available “to be safe.” Resist it. Use the largest powder size your geometry allows. Finer powder has dramatically more surface area — Type 4 carries roughly 20% more surface area per unit mass than Type 3, and Type 5 about 75% more — and surface area drives oxidation. More oxidation means a shorter shelf life, more random solder balling, and more graping (un-coalesced, grape-cluster joints). Type 5 and 6 pastes also cost more and demand tighter process control.

To match powder to your stencil, two numbers govern release. The 5-ball rule says the smallest stencil aperture dimension should be at least five times the diameter of the largest powder particle, so at least five spheres span the opening. Second, the area ratio — the aperture’s opening area divided by its wall area — should stay at or above 0.66 for reliable transfer; below that, paste clings to the aperture walls and you starve joints. Both calculations tie powder type directly to your PCB stencil design, which is why paste and stencil have to be specified together, never in isolation.

A wearables startup we worked with printed a dense 0201 layout with Type 3 paste to use up existing inventory. The boards passed automated optical inspection (AOI) and shipped — then a few thousand units in, field returns showed intermittent dropouts that traced back to starved joints on the smallest pads. The Type 3 powder could not release cleanly through apertures sitting near a 0.5 area ratio. Switching to Type 4 paste and reworking the stencil so every critical aperture cleared the 0.66 area ratio brought first-pass yield back above 99%.

How to Choose the Right Solder Paste, Step by Step

Treat paste selection as a sequence of constraints, not a single pick. Work through these in order:

1. Lock the alloy to your compliance and thermal limits. If you ship into RoHS markets, that means lead-free — SAC305 unless you have a reason to deviate. If you have heat-sensitive parts or warp-prone boards, evaluate low-temp Sn/Bi. If you are under a RoHS exemption and want the easiest process, leaded Sn63/Pb37 is on the table.
2. Choose the flux by cleaning and reliability needs. No-clean is the default for consumer and most commercial work. Specify water-soluble (and plan a DI wash) for high-reliability or heavily oxidized surfaces, and RMA where rosin residue is acceptable.
3. Set the powder type from your finest feature. Find the smallest aperture on the board, apply the 5-ball rule and the 0.66 area ratio, and pick the coarsest powder that still prints it cleanly — usually Type 4 today, Type 5 only if the geometry demands it.
4. Match viscosity and format to the application method. Stencil printing wants a higher-viscosity print paste; syringe and jet dispensing want lower-viscosity dispense grades. Confirm the paste ships in the format you will actually use — jar, cartridge, or syringe.
5. Qualify the specific paste against J-STD-005 and your line. Datasheet numbers — viscosity, slump, solder-balling, tack life — are a starting point, not a guarantee. Run test boards through your full print-place-reflow process and inspect against IPC-A-610 acceptance criteria before committing to production.

How to Apply Solder Paste: Printing, Dispensing, and Jetting

There are three production ways to get paste onto a board, plus manual application for one-offs.

Stencil Printing

Stencil printing is the volume workhorse: a laser-cut stainless-steel stencil sits on the board and a squeegee drags paste across it, forcing a controlled deposit through each aperture. A well-tuned printer runs squeegee speeds around 20–40 mm/s at a 45–60° blade angle and places paste on thousands of pads in one stroke. Stencil thickness — typically 100–150 µm — and aperture size set the deposited volume, and apertures are usually reduced 10–20% from pad size to prevent bridging. Print quality is where most assembly defects are born, so inline solder paste inspection (SPI) to verify deposit volume earns its keep on any serious line.

Dispensing and Jetting

Syringe dispensing pushes paste through a needle point by point — slower, but ideal for prototypes, rework, odd-shaped pads, and paste-in-hole work where a stencil cannot reach. Jet printing fires micro-dots of paste without contacting the board, which suits 01005 components, high-mix low-volume runs, and selective deposits on boards that already carry components. Both use lower-viscosity paste grades than stencil printing.

Reflow Brings It Together

However you apply it, the paste only becomes a joint in the oven, where reflow soldering runs the board through a profiled thermal ramp:

1. Preheat — ramp gradually to drive off solvents and bring the whole board up evenly.
2. Soak / activation — hold in a temperature band where the flux activates and reduces oxides on the powder and pads.
3. Reflow — spike above the alloy’s liquidus (roughly 240–250°C peak for SAC305) so the solder melts, wets, and coalesces.
4. Cooling — bring the board down at a controlled rate to set a fine-grained, mechanically strong joint.

One counterintuitive point on profiling: a faster ramp does not automatically reduce voiding. Pushing through preheat too quickly can seal the deposit surface before flux volatiles finish outgassing, trapping gas and making voids worse. Match the profile to the alloy and the paste maker’s recommendation, and verify it against IPC J-STD-001.

Solder Paste Storage and Shelf Life Best Practices

Paste is a perishable chemical, and storage quietly causes a lot of “mystery” defects. The flux activators are in constant contact with the powder, so even on the shelf they slowly attack the metal; given time, particles weld into clumps that raise viscosity and clog apertures. Cold slows that reaction down. The essentials:

• Refrigerate unopened paste at 2–10°C. Most no-clean pastes carry about a 6-month shelf life refrigerated (some formulations 6–12 months); water-soluble and fine Type 5/6 pastes run shorter because their chemistry and surface area react faster.
• Never freeze it. Below 0°C the flux and alloy can separate irreversibly.
• Temper before opening. Let a jar reach room temperature — about 4–8 hours for a 500 g container — before you break the seal, so condensation does not form inside and add moisture. Do not shortcut this with an oven or hot plate; uneven heating wrecks the rheology.
• Do not re-refrigerate opened paste. Once a jar is open, the damage from repeated condensation cycles outweighs the benefit of cold. Reseal, store it at room temperature, and use it up.
• Stir gently if it has been sitting. Thirty to sixty seconds in one direction with a clean non-metallic spatula re-homogenizes the paste without whipping in air.
• Run FIFO. Date every container on receipt, use oldest first, and never combine fresh and used paste in the same jar — used paste contaminates new.
• Mind stencil life and the print environment. A bead of paste on the stencil typically lasts one 8-hour shift; keep the print area around 22–26°C and 45 ± 5% relative humidity, and reflow printed boards within about 4–8 hours.

Common Solder Paste Mistakes (and How to Avoid Them)

This is the section to send a junior process engineer before their first build.

• Over-specifying powder fineness. Defaulting to Type 5 “to be safe” buys oxidation, solder balling, and short shelf life you did not need. Use the coarsest powder your apertures allow.
• Ignoring the area ratio. Apertures below a 0.66 area ratio starve joints no matter how good the paste is. Check it before you cut the stencil.
• Running the wrong reflow profile for the alloy. A leaded profile under SAC305 gives cold, dull, poorly wetted joints. Profile to the alloy’s liquidus.
• Chasing voids with a faster ramp. Slow the soak instead and let the flux outgas; speeding up traps it.
• Treating no-clean as universally safe. Residue under conformal coating, on ICT pads, or across RF nodes can bite. Clean or qualify for those boards.
• Printing with warm or expired paste. Paste straight from the fridge condenses moisture; paste past its date or left out causes graping and balling. Temper, date, and rotate.
• Specifying paste and stencil separately. The two are one system — the 5-ball rule and area ratio connect them.

Frequently Asked Questions About Solder Paste

What is solder paste made of?

Solder paste is roughly 88–91% solder alloy powder by weight, suspended in a flux vehicle of resin or rosin, solvent, activators, and rheology modifiers. The metal forms the joint; the flux cleans oxides and holds parts in place until reflow. Despite being about 90% metal by weight, a deposit is only about half metal by volume.

Is solder paste lead-free?

Most solder paste sold today is lead-free, driven by RoHS — typically SAC305 (tin-silver-copper). Leaded Sn63/Pb37 paste is still made and used under RoHS exemptions for aerospace, defense, medical, and certain industrial hardware, and for hand rework, where its lower melting point and wide process window make it easier to work with.

What is SAC305 solder paste?

SAC305 is the most common lead-free solder alloy: 96.5% tin, 3% silver, 0.5% copper, with a melting range of 217°C (solidus) to 220°C (liquidus). It is RoHS-compliant, wets well, forms strong joints on copper, and is the default for general SMT assembly — though it needs roughly 30–40°C hotter reflow than leaded solder.

Does solder paste expire?

Yes. Refrigerated at 2–10°C, unopened paste typically lasts about 6 months (some formulations 6–12). Once opened, use it within weeks, not months. Expired or mishandled paste oxidizes, causing solder balling, graping, and weak joints. Always check the date and run a test print before committing fresh stock to production.

Can you apply solder paste without a stencil?

Yes — syringe or needle dispensing places paste pad by pad, and is fine for prototypes, rework, and odd geometries. It is slower and less consistent than stencil printing and impractical for dense fine-pitch boards at volume. For anything beyond a few boards, or below about 0.5 mm pitch, a stencil pays for itself fast.

What temperature does solder paste melt at?

It depends on the alloy. Eutectic leaded Sn63/Pb37 melts at 183°C. Lead-free SAC305 melts at 217–220°C and reflows with peaks around 240–250°C. Low-temperature tin-bismuth pastes melt near 138°C. Always set your reflow peak above the alloy’s liquidus, per the paste datasheet.

No-clean vs water-soluble — which should I use?

No-clean is the default: no wash step, mild residue, longer shelf life, ideal for consumer and most commercial boards. Choose water-soluble when you need aggressive wetting on oxidized surfaces or zero residue for high-reliability work — but you must wash it off with deionized water, or the corrosive residue will damage joints.

Getting Your Solder Paste Selection Right

Solder paste looks like a commodity until a wrong choice surfaces as field returns. The discipline is simple: fix the alloy to your compliance and thermal limits, fix the flux to your cleaning and reliability needs, and let your finest feature and stencil set the powder type through the 5-ball rule and a 0.66 area ratio — then qualify the specific paste on your own line. Do that and most print and reflow problems never start.

Before your next build, do this:

• Pull your BOM, find the smallest pitch or component, and apply the 5-ball rule to set your powder type.
• Audit every critical stencil aperture’s area ratio and flag anything under 0.66.
• Confirm your reflow profile peaks above your alloy’s liquidus — never run SAC305 on a leaded profile.
• Check the fridge: verify it holds 2–10°C, date and FIFO your paste, and stop re-refrigerating opened jars.

If you would like a second set of eyes, send us your Gerber and BOM and our team will flag paste, stencil, and DFM risks before you build.