When Being Tense is Good
Stencil use leads to lower tension, and eventually misalignment. Is there a solution?
Anyone with even a modicum of understanding about the printing process knows that stencil (or screen) tension is integral to print integrity. While the amount of tension is important, what’s even more critical is that the proper tension is evenly distributed across the stencil. There is also some evidence that points to higher tension being more effective than lower tension, particularly for today’s finer-pitched devices and especially when there is an extremely high density of apertures. Take a stencil for one of today’s mobile phone printed circuit boards (PCB), for example. These stencils tend to have more holes than stencil, so tension is critical.
On average, tensions between 30 and 35 Newton are a good starting point for a properly tensioned stencil. If the tension changes over time – either from process use or post-process cleaning – the printing integrity will be affected. When apertures are filled, the board and the stencil come together like a sandwich. If the tension in any part of the stencil has diminished, as the print stroke begins, the stencil image moves and gets pushed away with the direction of the squeegee. As the apertures are being filled, there will be misalignment as the print stroke progresses and it becomes increasingly difficult to keep the print process in control.
When the board is released from the stencil, a well-tensioned stencil will produce a very controlled release. If the stencil has become baggy and less tense, the stencil will follow the board down with the table so, instead of a complete clean separation, some parts of the stencil will separate before others. In the high-density areas of the board – where there are many, many 0.4mm CSPs and hundreds of 0201s or 01005s – there is a large volume of solder paste that wants to stick on the apertures. This condition will tend to pull the stencil down, resulting in an uncontrolled release.
Traditionally, mesh mounted stencils have been the stencil architecture employed most frequently in electronics assembly. When using a metal mesh (as opposed to a polyester mesh) from which the stencil is suspended, extremely high tensions can be initially created. However, mesh mounted stencils tend to relax and lose tension as they are used and cleaned. Consider that the entire stencil is placed into the cleaning system that uses temperature and chemistry to clean off solder residues. Then, the stencil is dried with hot air. Cleaning and drying are inherently bad for the adhesive materials used to manufacture mesh mount stencils. This process results in varying coefficients of thermal expansion, which cannot only weaken the elasticity of the stencil and mesh, but can also cause the stencil image to shift. When processing fine-pitch devices, this is highly problematic.
So, what’s the solution? The best option would be to clean the foil only and preserve the integrity of the tension. This is precisely the idea behind many of the market’s frameless stencil tensioning systems. With these stencils, the foil is separate from the frame, which has many advantages. While some of these systems faced early challenges (operator injuries from sharp edges, for example), these obstacles have been overcome, and the popularity of frameless stencils has grown in recent years. There are many benefits to frameless stencil technology, including storage space savings, sustainability, lower costs over the long term and, most important, no loss of tension over time. The stencil foils are cleaned once removed from the frame, so there are no concerns about mesh or adhesive impacts. When placed into the frame, the foil is tensioned to the same level each time and, because there is no mesh to contend with, there is no loss of tension – even in high-volume manufacturing environments. In comparison, mesh mounted stencils used in high-volume processes need to be replaced an average of every two to four weeks.
In the past, frameless stencils provided tensions of 30 to 35 Newton, which was fine for standard SMT (0.5mm pitch and above) but not for today’s miniaturized devices. If using these systems for fine-pitch processes, a frameless stencil that can provide a tension of 40 Newton or greater is required for a robust process. Anyone considering a frameless stencil system needs to bear this in mind during the selection process.
The new challenges of miniaturization dictate now more than ever that stencil tension is consistent. Tighter pitches combined with thinner stencils and high-density apertures make stencil tension control and uniformity an increasingly critical parameter. In this case, tension is indeed a good thing.
Clive Ashmore is global applied process engineering manager at DEK International (dek.com); cashmore@dek.com. His column appears bimonthly.
