Step stencils

 

 

  Step stencils let you provide different volume of solder paste for the electronic assemblies whose components require different amount of solder paste on the one PCB during stencil printing.

• You can provide a bigger volume of solder paste for certain areas with using stencil made by Step-up technique ( thickening of certain stencil areas).

 

• or vice versa reduce the volume of solder paste for certain areas with using a stencil made by Step-down technique (thinning of certain stencil areas)

 

• If PCB has a thickened surface that prevents stencil from gasketing, you can use a special stencil with pockets compensating irregularities of PCB during paste printing.

  It's possible to use several levels of stencil thickness.

Our engineer will help you to prepare a step stencil project. It will be enough for you to highlight necessary component pads, which should be in certain area of thickness on the stencil and specify desired thickness for them.

  Minimal step for these stencils will be 0.08mm.

  Production time -  2-3 working days.

 

  

  Take a closer look at different examples of using step stencils.

Introduction:

  Initially, step stencils were used to reduce the stencil thickness for 0.64mm-pitch (25mil) QFPs and less. However, as SMT requirements became more complex and, consequently, more demanding, so did the requirements for complex step stencils. 

  Nowadays, step stencils allow applying: 

• Mixed-technology solder paste printing for:

a) through-hole \ SMT components

b) solder paste \ glue or solder paste \ flux

c) small-pitched components and small components \ normal-sized chip components 

• Lasercut step stencil is a good solution to achieve additional solder paste height on the pads of different BGA and other components.

• Special step stencils can be used for odd PCBs having some raised areas.

• Two-step printing process helps to solve the problem of  combined  printing for small and large components on the same PCB and different substances like solder paste, glue or flux in the same process time.

Let's look into detail at some ways of using step stencils.

 

Ceramic BGAs

Ceramic BGAs present a challenge to the SMT assembly process. Since high melting temperature (due to ceramic body, absorbing the heat) prevents the solder balls from melting at normal reflow temperatures, any slight coplanarity problem can result in an open contact to the CBGA balls. To prevent this problem it's desirable to print higher solder bricks on the CBGA pad. Typically, solder paste height of 0.18-0.2 mm is enough for this type of component.

  On the other hand, SMT components like 0.5mm-pitch QFPs and less, 0402 chip components are incompatible with an 0.2mm-thick stencil. Their aperture sizes are too small to achieve a good paste release.

An example of this stencil is shown in Figure 1.

Fig. 1 Step-up stencil for Ceramic BGA.

  The step-up stencil is made as a combination of basic 0.127 mm- thick stencil with thickening areas up to 0.2mm in the CBGA. Final electopoloshing is recommended for this stencil to achieve better solder paste release.

Another example of step-up stencil is shown in Figure 2.

Fig. 2 Example of step-up stencil for Ceramic BGA.

  The stencil thickness is 0.127 mm with thickness,increasing up to 0.2 mm under the BGA component.

  Typically raised areas are placed on the squeegee side of the stencil. This case is shown in two previous examples. 

  A general rule for spacing between the step edge and first aperture in the lower thickness area is 0.9 to 1.27mm per 0.03mm of step height.

  For example, if height of areas changes from 0.127mm\0.2mm to 0.073mm, the nearest aperture must be done not closer than 1.8-2.54mm from edge of the step area.

  If these rules are followed, metal squeegee blades should work with step stencils without any problem. Also another option is to use a soft polyurethane squeegee. 

 

Step stencils for mixed Through-Hole\SMT printing

  It's possible to use mixed solder paste printing for assembly through-hole \ SMD components in the same  cycle.

There are several ways to use stencils in this case :

• Solder paste printing of stencil with apertures slightly larger compared to the initial component pad size (overprint). This technique allows getting more paste on component pad after reflow, due to the fact that during melting paste accumulates on a pad because of the surface tension forces acting on the molten solder.
    There should be careful with this method when using lead-free solder pastes, due to the less ability of pulling the solder on the component pads.

• Using the stencil with a thickened area at the through-hole components area with slightly increasing the aperture (step-up and overprint).

• A two-stage stencil printing.In this case, a thicker stencil, that provides more solder paste is used at the second print.

Fig. 3 Step-up laser-cut stencil for through-hole connector.

  Figure 3 shows a stencil with raised area on the squeegee side for placing of output connector. The stencil has a thickness of 0.25 mm in the area of connector and 0.127 mm in the remaining areas. In this case, the printing direction of the squeegee is going over entire length of higher area and squeegee overcomes additional height of 0.127 mm. during printing.  

 

Step Stencils with relief pockets on contact side

• Raised via pads on PCB:

    If there are raised via pads on the board, they will prevent the stencil from smooth gasketing to the PCB. To achieve a good contact on PCB side of stencil, where there are a raised vias, it's necessary to have relief pockets, wherever there are raised vias. Typically, the relief pocket depth is half of a stencil thickness which is enough to compensate additional height on PCB. An example of such stencil can be seen in Figure 4.

  

  Fig. 4 Raised via pad relief step stencil.

• Barcode labels on the PCB

    Many printed circuit boards can have bar-coded labels mounted on the surface of the boards. If it is glued too close to the pads of components, it prevents the stencil from gasketing to PCB during printing. In this case, usage of stencil that has a relief pocket in the barcode area can solve this problem.

• Printed circuit board with build-up tracks

 Sometimes on PCBs possible options build-up tracks performed as an alternative of original PCB schematics, adjusting them after manufacture. Sometimes it may be graphite paths at the intersection where it was impossible to arrange any other junction ways for designing the tracks in a PCB copper layer.

  Such build-up tracks create extra thickness on the PCB surface, that's why on stencil special relief tracks are made for its compensation. An example of this stencil can be seen in Figure 5.

Fig. 5 Stencil with relief tracks.

 

Step stencils for two-stage mixed printing technology

• Technology of mixed printing for THT\SMT components

  If increasing apertures for THT components more then their original pad size (overprint) does not provide enough volume of solder paste during reflow, a thicker stencil must be used. At the same time, standard size components can't be used with a thick (0.4-0.5 mm) stencil and even lowering areas (step-down) from 0.5 mm to 0.127 mm is impractical. Alternatively, PCB may contain THT components located randomly and it is impossible to assemble them in one thickened area. What to do?

  In this case it's possible to use printing of solder paste in two stages.

 First we do a solder paste printing using 0,127-0,150 mm thick stencil . Because solder paste is applied under fine-pitch components \ small chips that require precise alignment of PCB and stencil, it is reasonable to use a large industrial printer with fiducial alignment and pneumatic 4-side tension stencil frame at this stage.

  The second printing step is done using a 0.4mm-thick stencil that has a relief pockets on PCB side in the places where solder paste was applied on the fine-pitch components pads at the first stage. It allows us to close a first-stage solder paste prints and apply thicker prints through the stencil at the second printing stage. As a result, we obtain a PCB with solder paste prints placed in random areas and having different height of paste.

  As a rule, the relief pocket should be 0.1 mm deeper than the height of the first stage paste prints.

  Because thicker stencil for larger components is used during the second print, and precision alignment is not required, this operation can be done manually on a simple manual stencil printer. In SMT manufacturing line it allows a quite complex technology of multilevel solder paste printing to be organized without unnecessary financial investments, combining the number and location of different-height solder paste prints in random way.

• Combined printing of Flip-chip\SMT components

   There are applications when it's necessary to print either solder paste or flux for flip-chip components on a PCB. After combined printing a joint reflow is done in an oven.

  Typically the stencil thickness  for Flip-chip components printing  is 0.03-0.05 mm, which is much too thin for usual SMT printing.

  Two-stage printing technology is the most suitable in this case. First flux prints through a thin (0.03-0.05 mm) stencil, and then, in the second stage, the solder paste is applied through 0.127 mm -thick stencil. A second stencil has relief pockets in the flip-chip area on the contact side. Figure 6 shows 0.127 mm thick stencil with 0.08 mm deep relief.

Fig. 6 Step stencil for printing with flux relief.

• Thick stencil with deep cavities under the glue

  By the time cycle of stencil glue printing is shorter than its applying by dispenser, because you need less time to migrate on each kind of PCB and the application process is performed in a single pass for the entire PCB at once, rather than individual glue points.

  Due to the difference in the physical properties glue gets on the board a little differently than the solder paste and may partially remain in the apertures of the stencil . Changing the aperture size the volume of glue getting on board of the same stencil thickness can be varied. Larger aperture will give over the adhesive by lifting the stencil , whereas glue will remain in smaller apertures and will be fed partly. Glue height versus aperture size for a 1.27 mm-deep relief pocket is shown in Figure 7

Fig. 7 Glue height versus aperture size for a 1.27 mm-deep relief pocket

  Figure 7_1 illustrates the fact that a certain amount of glue can remain in the stencil apertures, depending on their size.

Fig.7_1 Difference of heights of glue prints, depending from diameter of the stencil apertures.

  This option is very useful when the PCB contains components with different stand-off heights.

  For example, let's see the PCB with chip components having a stand-off of 0.1 mm and SOIC component with a stand-off of 0.4 mm. This is shown schematically in Figure 8. It is obvious that different volumes of glue bricks must be applied under these components. By using 0.4 mm stencil the aperture size is chosen to provide 0.15 mm of glue bricks height for a chip component, and 0.4 mm for SOIC component.

Fig.8 Chip components and SOIC with different stand-offs.

    During printing glue the stencils with relief pockets can be used. The main purpose of the relief pockets for glue is to reduce glue paths through the small apertures of thick stencil, providing the transfer of glue to the board under relatively low pressure of squeegee. Figure 8_2 shows adhesive pockets around glue apertures of stencil.

Figl.8_2 Relief pockets on stencil for accumulation of glue.

• Two-stage glue printing after solder paste application

  As previously mentioned, printing glue compared to dispensing is more convenient for large and complex printed circuit boards.

  We could use the next sequence:

1. First apply the solder paste on component pads

2. Then we apply adhesive, using a 0.5 mm- thick stencil with relief pocket depth of 0.4 mm on the PCB side of the stencil.

    The following figure illustrates a schematic cross structure of such glue stencil.

Fig.9 Glue stencil of 0.5 mm thickness with relief pockets for 0.4 mm-depth paste prints.

  Figure 10 shows the result of combining glue and solder paste printing for the component. Afterward the components are put in its places and a joint reflow is done in the oven. The result is components soldered and glued simultaneously in the one production cycle, it excludes the necessity of dividing these operations into different stages as well as usage of additional expensive equipment.

  As well as for two-stage solder paste printing, for glue printing a simple manual printer, which is almost always availible as an addition to automatic industrial printer at most SMT lines could be used.

Fig.10 Glue and solder paste prints for 0805 component.

• Stencils for special flex-rigid PCB

  Let's imagine that we have two rigid printed circuit boards connected by a flexible connector, raised above the PCB surface. This connector could be a problem for normal printing solder paste, since it prevents the stencil from tight gasketing to the PCB.

  The solution is to create the stencil that has a high relief pocket. Figures 12 and 13 show such stencil from both sides.

Fig. 12 View of the stencil with a relief pocket from PCB side..

Fig.12 View of the stencil with a relief pocket from squegee side.

  In this case special squeegee blade with a pocket is made for this stencil, because the step area is located along the whole stencil edge. Such blades are usually made of 0.2 mm-thick steel and can be ordered when you order the stencil.

•   Two-stage printing a very small and large components on the same PCB

  Often a problem occurs when you need stencil printing for example small 0.3 mm pitch BGA components or 01005 chips at the same time as large components such as QFP, SMT-connectors etc. The problem is shown in Figures 14 and 15.

Fig.14 Printing sequence for thick stencil.

   If a thick stencil is used, it provides a large solder paste prints because of a good paste release from large apertures. At the same time the small aperture release the paste not completely, since the ratio of the width of the aperture is wrong for this stencil thickness, which leads to clogging of the apertures, paste shortage at the contact pads and as a consequence, to dry soldering without good solder joints.

Fig.15 Printing sequence for thin stencil.

  From the other side, the thin stencil provides a good paste release for both large and small components. However, since the paste is insufficient for larger components, depleted brazing solder will be formed.

  Relation matrix of aperture size and stencil thickness is shown in Figure 16. It illustrates the acceptable thickness of stencils for 01005 components and 0.4mm microBGAs.

Fig.16.Relation matrix of aperture sizes and stencil thickness.

 One possible solutions for this complex print is a two-print stencil printing process where small components will be print first.

Fig. 17.  Example of  0.08 mm thickness stencil  for first stage of two-stage printing.

  After applying paste for small components through thin stencil we should do the second printing stage for other components. Because precise alignment of stencil is not required at this stage, this operation could be done on usual manual stencil printer, which is often available besides automatic one at many SMT lines.

Fig.18. A stencil with relief pockets for the second print.

 On the second stencil in all places where paste was deposited in first print, the relief pockets were made on the PCB side.  

 

This article is adapted translation of an article by William E. Coleman of the company PHOTOStencil, published in the journal SMTonline.com in August 2011.