Solder paste is the lifeblood of surface-mount assembly. This mixture of tiny solder particles suspended in flux determines the quality of every solder joint on your board. Choosing the right paste type, particle size (mesh), and managing its storage correctly can mean the difference between a board with zero defects and one with bridges, voids, and cold joints. This guide covers solder paste selection and handling for Indian PCB assembly operations.
Table of Contents
- Paste Composition
- Solder Alloys
- Particle Size (Mesh) Selection
- Flux Type Classification
- Stencil Printing
- Storage Requirements
- Common Paste-Related Defects
- Frequently Asked Questions
Paste Composition
Solder paste is approximately 85-90% metal particles by weight (45-55% by volume) suspended in a flux medium:
- Metal particles: Spherical solder alloy particles providing the metallic bond
- Flux: Chemical medium that removes oxides, promotes wetting, and holds particles together
- Viscosity modifiers: Control the paste’s flow properties during stencil printing
- Activators: Chemical compounds that clean the copper surface during reflow
Solder Alloys
| Alloy | Composition | Melting Point | Use Case |
|---|---|---|---|
| Sn63/Pb37 | 63% tin, 37% lead | 183°C (eutectic) | Hobby, prototype, non-RoHS |
| SAC305 | 96.5% tin, 3% silver, 0.5% copper | 217-220°C | Lead-free production standard |
| SAC387 | 95.5% tin, 3.8% silver, 0.7% copper | 217°C | Higher reliability applications |
| SnBi | 42% tin, 58% bismuth | 138°C | Low-temperature assembly |
For Indian commercial products: SAC305 is the standard lead-free alloy. For hobby and prototype work: Sn63/Pb37 (leaded) is easier to work with and produces better-looking joints. For export to EU: SAC305 is mandatory (RoHS compliance).
Particle Size (Mesh) Selection
| Type | Particle Range | Best For |
|---|---|---|
| Type 3 | 25-45µm | Standard SMD (0603 and larger, 0.5mm pitch and above) |
| Type 4 | 20-38µm | Fine-pitch (0.4-0.5mm pitch QFP, 0402 components) |
| Type 5 | 15-25µm | Ultra-fine-pitch (0.3mm pitch BGA, 0201 components) |
| Type 6 | 5-15µm | Micro BGA, chip-scale packages |
Type 3 is the default for most Indian assembly operations. Switch to Type 4 for designs with 0.4mm pitch QFP or 0402 passives. Type 5 and above are needed only for advanced HDI assemblies.
Rule: At least 4-5 solder particles must fit across the smallest pad aperture. If the stencil aperture is 0.25mm wide, the maximum particle size must be below 50µm (Type 3 is borderline; Type 4 is safer).
Flux Type Classification
| IPC Class | Name | Residue | Cleaning Required |
|---|---|---|---|
| ROL0 / ROL1 | Rosin (no-clean) | Clear, non-corrosive | No (cosmetic clean optional) |
| REL0 / REL1 | Resin (no-clean) | Clear to amber | No |
| ORH1 | Organic acid (water-soluble) | Corrosive if not cleaned | Yes (water wash mandatory) |
No-clean paste (ROL0/ROL1) is the standard choice for most applications — it eliminates the cleaning step, saving time and cost. Use water-soluble paste only if you have a reliable cleaning process (DI water + ultrasonic cleaner) and need the highest solderability for difficult joints.
Stencil Printing
- Stencil thickness: 0.12mm for standard 0.5mm+ pitch. 0.10mm for 0.4mm pitch. 0.08mm for 0.3mm pitch
- Aperture design: 1:1 ratio for most pads (aperture matches pad size). Reduce to 80-90% for thermal pads to prevent voiding
- Area ratio: Aperture area / aperture wall area must be above 0.66 for reliable paste release. Below this, paste sticks in the aperture
- Squeegee pressure: 0.5-1.0 kg per cm of blade length. Too much pressure scoops paste out of apertures
- Print speed: 20-50mm/second for standard, slower for fine-pitch
Storage Requirements
- Refrigerated storage: 0-10°C (standard refrigerator). Extends shelf life to 6-12 months depending on paste formulation
- Before use: Remove from refrigerator and allow to reach room temperature (25°C) for 2-4 hours before opening the jar. Opening cold paste causes moisture condensation that degrades paste quality
- Working life: Once dispensed on the stencil, paste is usable for 4-8 hours (varies by formulation). After that, the flux solvents evaporate and paste becomes too dry
- Jar handling: Do not stir vigorously — this shears the flux medium. Mix gently with a spatula before dispensing
- Never return used paste to the jar: Contamination from the stencil degrades the unused paste
Common Paste-Related Defects
| Defect | Cause | Solution |
|---|---|---|
| Solder bridges | Too much paste, wrong mesh size | Reduce stencil thickness or aperture size |
| Insufficient solder | Clogged stencil, low paste volume | Clean stencil, increase aperture |
| Solder balls | Paste on solder mask, splashing | Improve stencil alignment, clean board |
| Tombstoning | Unequal paste on pads, uneven heating | Balance pad sizes and paste volume |
| Voiding | Flux outgassing trapped under component | Optimise reflow profile, reduce paste on thermal pads |
Frequently Asked Questions
Can I use solder paste for hand soldering?
Yes, for small quantities. Apply paste to pads using a syringe or toothpick, place components, and reflow with a hot air station or hot plate. This is common for prototype assembly when you do not have a stencil. Paste applied manually is less consistent than stencil-printed paste but works for prototypes.
How do I choose between leaded and lead-free paste?
For prototypes and hobby: leaded (Sn63/Pb37) is easier — lower temperature, better wetting, shinier joints. For products sold commercially in India or exported: lead-free (SAC305) is recommended for RoHS compliance and customer expectations.
Where can I buy solder paste in India?
Small quantities (30-50g syringes): Amazon India, Robu.in, Zbotic.in. Production quantities (500g-1kg jars): Indium Corporation, Alpha Assembly Solutions, Kester — available through their Indian distributors. Budget option: Mechanic brand paste from AliExpress/Amazon.
Get soldering supplies, test equipment, and assembly tools at Zbotic Soldering Tools — delivering across India.
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