Chapter 4 of our comprehensive guide to stencils has been developed to help you understand the print cycle and the factors that can affect board assembly.

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4.0 Printing parameters

4.1 The printing cycle

A printer must be able to carry out several operations to complete the print cycle, which comprise:

  1. Transporting the substrate into position and contacting the stencil
  2. Alignment of stencil to board
  3. Application of solder paste – enabling paste roll and aperture filling
  4. Controlled separation of the substrate from the stencil
  5. Inspection of solder paste deposits (optional)
  6. Under screen cleaning to remove stray solder balls and contamination (after predetermined number of cycles)

The printing cycle time is an important factor in determining the line capacity or throughput. Typically, the printing cycle is completed between 9 seconds – for mass manufactured items such as mobile phones – up to approximately 25 seconds for small batch runs involving fine pitch. Today, the more significant factors are pick
and placement and reflow times. Often separate offline operations are used for inspection to avoid delays to the total cycle time.

The sequence of printing operations is as follows:

  1. PCB enters printer and travels to board stop.
  2. PCB is clamped.
  3. Printer camera moves to fiducial 1 and locates its position
  4. Printer camera moves to fiducial 2 and locates its position
  5. Camera moves to rest position away from printing operation
  6. Stencil is aligned
  7. PCB support/platen moves up and PCB contacts underside of stencil
  8. Squeegee operation commences
  9. Board support moves down
  10. Board is unclamped and exits machine on the conveyor

This description of a printer integrated into a production line also applies to standalone printers in respect of board fixturing, alignment of substrate to stencil, board separation and cleaning.

There are many variations in the way mounting, alignment and cleaning are implemented, and standalone or shuttle printers will also handle boards manually rather than automatically.

Printers can vary from the simplest hinged frame and platen arrangement, where the printing is achieved using a manual squeegee blade, through semi-automatic printers offering control of the squeegee pressure and print stroke, to fully automatic printers that include all the features required of a flexible inline machine. In essence, the finer the component pitch being printed combined with the quantity being produced and the quality standard offered defines the type and attributes of the printer required.

printers

4.1 Board Mounting

The PCB or substrate is usually transported into and out of the printer by conveyor belts. The PCB arrives in the working area and is stopped in the desired position using either mechanical end stops or an optical sensor. It is essential that the PCB is clamped rigidly in position to prevent lateral movement, whilst also being supported to resist the downward forces during the print stroke, which would otherwise lead to warping and solder bleed under the stencil.

Underboard support can take several forms, including:

  • A dedicated tooling plate with dowel pins to align the board
  • A matrix of manual or programmable universal mounting pins

pin board

The exact solution chosen will depend on the application, but providing adequate support during second-side printing can be problematic, especially when the first-side assembly is densely packed with components, or the board is thin or flexible. In such applications a dedicated tooling plate can be employed which is machined to accommodate the components. This often provides improved support which is better than a bed-of-nails fixture, especially around the extents of the substrate.

A 3-D profiled aluminium nest plate can be created complete with supporting pillars and routed areas to accommodate the first-side components whilst offering a flat substrate to the underside of the stencil.

nest for printing

4.3 Board Clamping

Clamping the PCB to the conveyor rails can be achieved mechanically, using edge clamps which are thin enough not to impede squeegee travel (and which are orrespondingly sharp!). It must be remembered that using clamps on PCBs where the surface mount features are too close to the edge of the board can and does lead to bleeding,
bridging and or insufficient paste deposits.

clamping foils

Some PCB designs do not have salvage or snap off regions whilst also containing fine pitch components very close to the edge of the PCB itself. Although with the older shuttle printers this is not likely to be a problem, modern inline printers utilise clamp foils to retain the PCB in registration throughout the print cycle. Even
with these thin foils it is not possible to ensure the stencil gaskets to the PCB – thereby creating paste release problems, paste retention or stencil clogging and also paste bleeding bridging and short circuits.


Where possible ensure the PCB design has a snap-off or salvage area whenever fine pitch components are located in close proximity to the finished PCB edge. The red shaded area shown in figure 4.7 represents approximately 2-4mm where the stencil cannot achieve good contact with the substrate.

To ensure the stencil is able to seal to the solder pads at the edge of a PCB, when using in-line printers the underside of the stencil can be relieved to accommodate the clamping foils.

The part etched nests on the underside of the stencil enable fault free printing.

nesting of clamping foils

4.3.1 Vacuum Tooling Plates

Other holding options include the use of vacuum tooling plates – where the substrate is held against the tooling plate with vacuum assistance. This method can be used to assist in overcoming flatness problems associated with warped boards (although this task is largely carried out by the downward pressure of stencil and squeegee
blade).

4.3.2 Pin Bar Arrangements

Semi-automatic printers often have simple pin bar fixtures to retain the PCB throughout the transport and printing modes. The inherent accuracy of these printers is less than their equivalent automatic counterparts since drilled holes in PCBs used with the pins as shown in figure 4.11 often tend to be ± 50 – 100µm of the nominated
dimension required.

4.4 Image Positioning

Examples of image positioning are shown below:

4.4.1 Roll on/ roll off distance

It is important to provide the solder paste with every opportunity to roll; as such a distance at the start and end of the printing stroke, known as the roll on / roll off distance should be accommodated.

The RO/RO distance should be between 30-40mm. Its purpose is to provide sufficient momentum to the paste to promote good paste roll, which enables aperture filling.

roll on/ roll off

Squeegee separation also must be considered. This is a fixed dimension and on most modern printing machines is between 40 and 50 mm.

squeegee separation

4.5 Alignment

Alignment accuracy is critical to achieving success with the printing process. Positioning the board accurately and providing alignment repeatability to align the
copper features of the PCB pattern with the apertures in the stencil is the minimum required in this operation. This requires three adjustments (X, Y, Ø): since the
range of angular adjustment is small, alignment is often implemented using X1, X2, and Y adjusters as shown in figure 4.17.

Stencil alignment mechanism

The alignment of PCB pads to stencil apertures requires adjustments to X and Y axis controls as shown in figure 4.18.

Adjustment to X and Y axis controls

Final adjustments may involve an angular or theta adjustment where the front and rear X axis controls are used.

final adjustments

4.5.1 Vision Alignment Systems

Manual adjustment has now largely been replaced by vision systems using a CCD camera to image fiducial marks on the board. A number of different shapes of fiducial have been used in the past, although the industry tends now to use either a solid filled circle or diamond between 1mm and 3mm in diameter. It is essential to ensure the fiducials are unique so that the recognition factor involved in locating them with a vision system presents minimal confusion or problems. When repeatability is required, assisted-vision systems are a necessary part of any printer. There are several systems that can be used to achieve good alignment:

• Basic look-down camera systems used on semi-automatic printers that store the image of the features on a PCB and subsequently compare the printed deposit positions with the original copper pad
• Automatic fiducial recognition using either look-up / look-down camera technology – where the fiducials on the PCB and their counterparts on the stencil are compared and “best fit” alignment is achieved, using split
field prisms
• A look-down / look-down camera that also offers subsequent printed deposit inspection

Down-looking camera

In many systems, the camera is inserted between the stencil and the PCB, looking down onto the PCB as well as up onto the underside of the stencil to view the fiducials
simultaneously. The camera is free to rove beneath the stencil as it registers the fiducials and then it returns to its rest position away from the vertical motion of the printer rails.

Vision alignment system

Some printer manufacturers adopt different methods of employing cameras to recognise the fiducials including precision look-down / look-down cameras that store an image of the PCB fiducial or pad to compare with the corresponding stencil feature.

Fiducial identity can be referenced to either a lighter background, as in the case of the stencil or a darker background as on a PCB or substrate. Printer camera recognition systems are used to calculate the fiducial’s position, by scoring against target/accept scores. The ideal situation would result in a perfect match.
Image alignment accuracy depends on:

  • Optical and incident lighting system employed in the printer
  • Resolution, in terms of pixels into which the PCB image is converted by the camera
  • Algorithm used to determine the necessary location information from the board image

4.5.2 Alignment Compromises

Environmental variability – such as differences in temperature and humidity during production – can lead to small differences in dimensions, particularly on large boards, so there is no absolute guarantee that the stencil will exactly match the board even though these have been generated from the same CAD artwork.

Because of minor variability, the best an alignment system can do is to provide perfect alignment between the stencil and the corresponding feature on the PCB at one point, and to minimise the errors at all others. Vision alignment systems look at two or more fiducials, and then correctly align the fiducials to the line joining them
averaging the error along this line as shown in figure 4.24.

Averaging errors in fiducial positions

The one point of perfect match is somewhere between the two fiducials. Often this point is midway between them, but some boards may have non-central areas which are particularly difficult to print. In such cases, it is possible to ‘weight’ the fiducial correction, so that the optimum alignment occurs at the desired board location. In all cases, however, exact alignment in X, Y and theta, is only achieved at one place, and the inaccuracy increases with distance from this point.

4.6 Environmental Conditions

If possible, temperature should be kept constant within close limits: 21°C ±2°C is a typical specification. This is primarily in order to avoid any effect on the process resulting from changes in the viscosity of the solder paste, but also helps keep alignment consistent by maintaining constant stencil dimensions.

4.7 Squeegee Blades

Squeegee blades are effectively the paste delivery mechanism and to ensure the solder paste fills the finest of stencil apertures several factors need to be considered. These are covered in more detail in chapter 5 in this series.

Squeegee blades

4.8 Speed

Printing speed needs to be considered carefully. The first consideration should always be the finest aperture present on the stencil, parallel to the direction of print. Printing with an excessive speed may cause incomplete or inadequate aperture filling resulting in inconsistent printed deposits.

Always consult the paste manufacturer’s recommendations with regard to printing speeds required for their products. Equally poor results can be obtained by printing high speed paste at lower speeds – where the rheological properties of the paste do not reach the optimum conditions – as printing lower speed pastes above the
recommended speeds.

4.9 Pressure

Squeegee pressure depends mostly on length of blade being used. Blade lengths should be the equivalent of the board width +25mm at each end. If the squeegee blade employed is longer than this dimension it is often not possible to achieve good printed results without increasing the pressure significantly. As a good rule of thumb
0.5-1.0kg / 50mm should satisfy most solder paste printing requirements. Beware of using excess pressure as inconsistent printed results and under-stencil contamination usually occur.

4.10 Stencil Cleaning

Stencil cleaning may not be the most glamorous part of the printing process but is nonetheless essential. Cleaning is required at the end of printing when the stencil is removed from the printer, to remove any accumulations of solder paste from the aperture walls and the top and bottom stencil surfaces before the deposits harden. It is also necessary as an in-process activity to ensure printed deposit consistency. The frequency of under-stencil cleaning will depend on the finest pitch component present as much as the quality of the PCBs being printed, together with the accuracy of the printer.

Stencil cleaning frequency can be reduced by using nanocoated stencils. This is covered in more detail in chapter 3 in this series.

The underside of the stencil must enable gasketing to the copper features below or it will gradually acquire solder paste through:

  • Stencil bleeding
  • Misalignment
  • Poor paste release

The rate at which this happens will depend on the print parameters selected as well as the stencil technology, substrate condition, paste rheology and the prevailing environmental conditions.

Solder paste can accumulate on the underside of the stencil because small amounts of paste bleed resulting from an imperfect seal between the stencil and board. This phenomenon is made worse by misalignment, using squeegee pressures outside the process window or by poor release of paste from the stencil.

Periodic cleaning of the underside of the stencil is particularly important for fine-pitch applications, because even a small degree of contamination of the substrate by solder balls or flux from the solder paste will degrade the printed results through smearing. If the contamination is not removed, the resulting print smearing increases the incidence of bleeding, bridging and solder shorts or solder balling.

Paste clogging
Cleaning can be carried out by hand or completed automatically. Programmable in-process stencil cleaning can be built into modern automated stencil printers, while separate automatic spray-cleaning tanks may be used for stencils after printing.

Automatic stencil cleaners are designed to enable unassisted cleaning of the underside of the stencil at user programmable intervals (typically after a predetermined number of print cycles). The frequency of under-stencil cleaning will depend on whether the stencil is nanocoated, the alignment of the stencil to the PCB and also the environmental conditions.

Typical systems for cleaning the stencil underside use a lint-free wipe, running between supply and take-up rolls, so that an unused area of the paper is pressed against the underside of the stencil. Contaminants and paste removed from the stencil are trapped on the material roll.

Under-stencil cleaning mechanism

On some machines, this operation can be run dry or wet, using a cleaning fluid that is sprinkled onto the absorbent paper. Machines can also be programmed for different combinations of wet and dry wiping, using wet cleaning to loosen any dried solder paste residues.

The cleaning process can also be assisted by vacuum, which can help remove solder paste from stencil openings and improve the clearance of partially clogged apertures. The vacuum system operates in conjunction with twin blades on the under-stencil wiper to draw contaminants into the absorbent paper below the stencil surface.

4.11 Paste Conditioning

Paste needs to be stored correctly and allowed to reach operating temperature before the container is exposed to (potentially moist) air. Preparation for use should also include a degree of ‘paste conditioning’ to ensure that:

  • The paste is homogenous – during extended storage, solder spheres and flux may separate. Ideally, paste should be rolled slowly and continuously, which is especially difficult if the paste has been stored refrigerated to extend shelf life and is not brought to ambient temperature in a controlled manner
  • Initial folding or stirring of the solder paste does not introduce or entrap air
  • Once the solder paste has achieved a stable rheological state it is essential that it rolls or flows to fill the apertures, and then reverts to provide solid solder brick deposits before separation of the substrate from the stencil.

One technique available on the printing machine itself is the knead function where the paste is moved backwards and forwards over a portion of the stencil containing no apertures until suitable paste rolling action is achieved.

4.12 Print Quality

This usually involves a visual assessment of selected paste deposits to offer a quick and helpful guide to whether the process is under control. Good results can be obtained with relatively low magnification (×4 to ×10) using a magnifier or projection microscope, as these allow the whole area to be scanned relatively quickly. So, what should you be looking for?

A working definition of acceptable print quality is one that has good definition and registration without any defects such as slumping, scavenging, bridging and peaking. These defects are covered in more detail in chapter 6 in this series.

4.12.1 Paste Measurement

Measurement of the paste deposit is crucial to quality control, there being two significant aspects:

  • Has the whole of the intended pattern been printed successfully?
  • Have you achieved the volume of paste deposits required for the component population? (For which paste height and area are useful measurements).

The methods of evaluating a printed substrate vary between fully automated inspection, both for coverage and paste height, and occasional operator visual checks. Today, with the increasing use of smaller components, often with terminations below their bodies, there is a trend towards implementing automatic checks at the end of the
print cycle, using either the printer itself or a separate machine.

Until recently inspection of the paste deposits using the printer camera systems impacted or limited the required printing cycle times. But advances in technology – employing the existing sophistication and speed of the optical arrangements, combined with software – make it possible to complete a post print inspection in under a minute.

Optical inspection for coverage has in the past relied on there being a visual difference between a pasted and bare pad: this is very easy when printing onto a nickel-gold finish, but significantly more difficult when printing onto solder surfaces.

A ‘z-check’ for paste height can be carried out easily with a light-section microscope or laser equivalent. As shown in figure 4.29, the height of the print, and some information on the topography of the surface, can be gained using oblique illumination through a slit, and viewing from above.

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