Solder Paste Printing Process Overview

The solder paste printing process typically involves the following steps:

1. A stencil with apertures corresponding to the SMT pad locations is aligned over the PCB
2. Solder paste is dispensed onto the stencil surface
3. A squeegee blade is used to drag the paste across the stencil, forcing it through the apertures and onto the PCB Pads
4. The stencil is lifted away, leaving brick-shaped solder paste deposits on the pads

Key process parameters that influence print quality include:

• Squeegee speed and pressure
• Stencil material and thickness
• Aperture size, shape and wall smoothness
• Solder paste rheology (flow properties)
• Paste roll diameter and print direction
• Print gap between stencil and PCB
• Cleaning frequency of the bottom of the stencil
• Environmental factors like temperature and humidity

Solder Paste Inspection Methods

There are several methods commonly used to inspect and test solder paste prints to ensure they meet quality requirements:

2D Automated Solder Paste Inspection

2D automated solder paste inspection (SPI) systems use cameras and image analysis to rapidly measure the area, height, volume, and other characteristics of solder paste deposits across the entire PCB after printing. Key capabilities include:

• Measuring paste coverage (% of pad area covered)
• Detecting bridging or solder paste connecting adjacent pads
• Measuring x-y offset of prints relative to pad center
• Detecting missing or incomplete prints

2D SPI allows sampling rates up to 100%, inspecting every pad on every board if desired. Defects can be identified and the printing process adjusted in real-time. However, 2D systems cannot directly measure solder paste volume or height. Volume is estimated based on the measured print area and stencil thickness.

3D Solder Paste Inspection

3D SPI systems project structured light patterns onto the solder paste and PCB and use cameras to analyze the light distortions to construct a topographical map of the printed deposits. This allows direct measurement of solder paste height and volume in addition to the 2D measurements. 3D systems are slower than 2D and more expensive, so 100% inspection is usually not practical. Key advantages over 2D SPI include:

• Direct solder paste volume measurement (not just area)
• Detecting print volume variation across large pads/apertures
• Detecting paste slumping/spreading beyond the print area
• Detecting subtle variances in paste rheology

Cross-Sectioning

Cross-sectioning involves cutting out small sections of sample PCBs and examining the cross-section under a microscope to evaluate print and stencil characteristics not visible from above. Parameters often assessed include:

• Actual paste height vs stencil thickness
• Paste adhesion to pad (vs stencil)
• Print definition and edge quality
• Stencil aperture wall smoothness
• Gasketing between stencil and PCB

Cross-sections are destructive, so are limited to a small sampling of boards, but provide information not obtainable any other way. Sections can be further inspected after reflow to evaluate Solder Joint Quality.

Visual Inspection

Manual visual inspection with the naked eye or basic optical magnification is still used to quickly identify gross defects or stencil/squeegee related issues like:

• Poor stencil-to-PCB gasketing/sealing
• Stencil blockage or damage
• Squeegee wear, damage or contamination
• Bridging, smearing or missing prints
• Solder paste contamination or abnormalities

Visual inspection is qualitative rather than quantitative but allows rapid checking of the printing process. It complements automated inspection.

Solder Paste Print Quality Metrics

Some of the most important metrics used to quantify and control solder paste print quality include:

Transfer Efficiency

Transfer efficiency (TE) is defined as the ratio of the printed solder paste volume to the stencil aperture volume:

TE (%) = 100 x (Printed Paste Volume) / (Aperture Volume)

If the solder paste releases completely from the aperture walls, the deposit will have a volume equal to the aperture volume and the TE will be 100%. In practice, TE is typically in the 60-90% range.

Low TE can result in insufficient solder volume and open solder joints. High TE indicates overly large deposits that could cause bridging. Consistent TE across different sized apertures is desirable. TE tends to increase with smaller apertures.

Print Volume/Height

Print volume is the measured or calculated volume of each solder paste deposit. Insufficient volume can cause open circuits while excess volume promotes bridging, balling and tombstoning defects. Typical solder paste height is 100-150 μm.

Print height and volume are ideally measured directly with 3D SPI. If only 2D SPI is used, volume may be estimated as:

Estimated Paste Volume = (Measured Print Area) x (Stencil Thickness) x (Typical TE%)

Area Coverage

Area coverage is the percentage of the solderable pad area covered by printed solder paste. Typical targets are 60-90% coverage, with some types of components like BGAs requiring up to 100%.

Low area coverage may not provide sufficient solder or adhesion strength. Excessive coverage promotes bridging between pads. Coverage requirements depend on the pad and aperture size/shape and component type.

Print Offset

Print offset is the x-y deviation of the center of the printed solder paste deposit from the center of the pad. Typical targets are <25% of the pad width. Excessive offset can cause open solder joints or tombstoning, especially on small chip components.

Stencil Design Considerations

Stencil characteristics have a huge influence on solder paste print quality. Important stencil design parameters include:

Stencil Thickness

Stencil thickness determines the maximum solder paste volume. Thinner stencils are used for fine-pitch components to minimize bridging. Typical stencil thicknesses range from 100-150 μm (4-6 mils). Thicker stencils are needed for larger components with high solder volume requirements.

Aperture Size and Shape

Stencil aperture size and shape should be optimized based on the PCB pad size/shape, component type, and desired solder volume and area coverage. Key aperture parameters include:

• Width and length (or diameter for round apertures)
• Aperture-to-pad size ratio
• Area ratio (aperture area divided by sidewall area)

As a general rule, aperture width should be at least 4-5 times the stencil thickness to ensure good paste release. The table below shows typical minimum aperture dimensions:

Stencil Thickness	Minimum Aperture Width
100 μm (4 mil)	350 μm (14 mil)
125 μm (5 mil)	500 μm (20 mil)
150 μm (6 mil)	650 μm (26 mil)

For fine-pitch components, the aperture is often narrower than the pad to avoid bridging. Home plate or window-pane shaped apertures can increase paste volume while minimizing bridging. Aperture area should be reduced at the PCB edge to prevent paste from squeezing out of the stencil-PCB gasket.

Stencil Material

The most common stencil materials are laser-cut stainless steel and electroformed nickel. Electroformed stencils have smooth aperture walls that promote good paste release, and work well for fine-pitch applications. Laser-Cut Stencils are lower cost and suitable for most other applications.

Stencil materials should have low adhesion to solder paste, adequate strength and stiffness, thermal stability, and corrosion resistance. Nano-coatings are sometimes applied to stencil undersides to improve paste release.

Troubleshooting Solder Paste Print Defects

Some of the most common solder paste print defects and their typical causes include:

Bridging

Bridging is when solder paste connects adjacent pads, causing a short circuit. Causes include:

• Excess paste volume
• Paste slumping due to low viscosity
• Stencil apertures too wide or too close together
• Insufficient gasketing between stencil and PCB
• Worn or damaged squeegee blade

Solder Balls

Solder balls are small spheres of solder that form on the PCB surface due to:

• Solder paste sticking to the stencil aperture walls
• Excess paste printed beyond aperture area
• Dried out or contaminated paste
• Paste temperature too high

Poor Print Definition

Poorly defined prints with ragged edges or voids can result from:

• Poor paste roll during printing
• Dried out or cold paste
• Damaged stencils
• Uneven or worn squeegee
• Warped PCBs

Insufficient or Excessive Solder Paste Deposits

Solder paste deposits that are too small/thin or too large/tall are usually caused by:

• Stencil thickness not matched to pad size and component
• Stencil aperture size too small or large
• Squeegee pressure too low or high
• Paste viscosity too high or low
• Incorrect print gap between stencil and PCB

Frequently Asked Questions (FAQ)

What is the optimal solder paste print volume?

Optimal print volume depends on the component size, type and pitch. In general, fine-pitch components like 0201 chips need around 400-600 μm3 of solder paste per pad. Larger 1206 chips need 20,000-30,000 μm3. BGAs typically require 80-100% pad coverage.

How often should stencils be cleaned?

Stencils should be underside cleaned at least every 4 hours of printing to prevent aperture clogging. Stencils should be fully cleaned at least once per shift. More frequent cleaning may be needed for fine-pitch stencils.

What is the shelf life of solder paste?

Refrigerated solder paste typically has a shelf life of 3-6 months depending on alloy and flux type. Paste starts degrading within 8 hours once opened and exposed to air. Use paste within 4 hours of removing from the refrigerator.

How critical is the squeegee material, hardness and edge quality?

Squeegee blades should have a sharp, smooth edge free of nicks or cuts. Harder squeegee materials like stainless steel maintain their edge longer than softer polyurethane. A dull or damaged edge will cause uneven paste distribution and smearing.

What is the best method for testing solder paste quality?

A combination of automated 2D/3D SPI for measuring print volume, height and area; cross-sectioning to evaluate print and stencil characteristics; and visual inspection for quick qualitative assessment provides the most comprehensive evaluation of solder paste print quality.

By carefully monitoring and controlling the solder paste material and printing process parameters, and frequently testing prints to ensure all quality metrics remain within spec, a high level of solder paste print consistency and PCB Assembly yield can be achieved.