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5 Best Tips to Create Openings on PCB Stencils in 2021 - 100% Useful

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Wed, Jun 23, 2021

SMT stencil

PCB stencils also called SMT stencils are necessary tools for SMT assembly. Usually, when you order printed circuit boards, the PCB stencils are also manufactured with the boards. Technically, manufacturing the PCB stencils is not complex at all - openings are created on the stainless steel with lasers according to PCB pads in a 1:1 ratio. However, in the past 22 years, PCBONLINE has had so many times battling with the PCB stencils openings. Techniques and tips are necessary, and PCBONLINE grabs them through the practices.

The common reason for the above tips is that PCBONLINE needs to avoid the  stress concentration of the solder paste. During the PCB assembly process, the solder paste changes its state from the solid to the liquid and finally to the solid-state. Due to the stress concentration, the solder paste in the liquid state gathers. As a result, if the original solder paste on the PCB pad is long and thin, it becomes short and thick when it melts and solidifies. In this way, the tin is not enough for the circuit connection.

stress concentration

Explaining this, you may understand PCBONLINE's above tips for creating openings for PCB stencil manufacturing. For your better understanding, below are the explanations for the tips.

Details of Tip 1 -

ICs/QFP (quad flat packages) are of the rectangle shape so the PCB stencil's openings for their PCB pads should also be long and thin with a small width. If the stencil opening is exactly the size of the PCB pad, the solder paste sieved from the opening to the pad shrinks and becomes shorter than the pad, which will cause poor welding. To avoid the effect of the stress concentration, PCBONLINE designs two round ends of the IC/QFP stencil opening. In this way, when soldering, the solder paste is applied to the IC/QFP pad perfectly.

Details of Tip 2 -

BGAs have a round shape so a BGA's PCB pad is also round. PCBONLINE designs the SMT stencil’s opening to be a square with a side length equal to the radius of the round pad. Due to stress concentration, the solder paste gathers on the round BGA pad perfectly. As for the 0402 and 0201 packages components, their pads are so small, and if the stencil openings are of the same size, there can be little solder paste on the pads. So PCBONLINE designs their stencil openings to be square and a little larger than than the pads. In this way, the 0402/0201 pads can get enough solder paste to fully connect the components with the PCBs.

Details of Tip 3 -

thermal pad

The QFN grounding pads usually are the thermal pads as above. Due to stress concentration, if the stencil opening size is 1:1 to the thermal pad, the solder paste shrinks and doesn't match the pad. So PCBONLINE leaves 0.5mm's space from the pad's outer edge to the opening's outer edge. When the solder paste shrinks, it matches the thermal pad. Another reason for the extra 0.5mm's space is to prevent the tin ball residue. When the solder paste solidifies, the extra tin balls won't fall to other PCB pads or components to cause short circuits.

Details of Tip 4 -

right-tangle IC assembly

The USBs, mini USBs, compact flash (CF) cards, storage device (SD) cards, and some ICs (shown in the above image) have PCB pads with right tangles. PCBONLINE designs the stencil openings to be 1.2 times longer than these pads. The purpose is to allow sufficient liquid tin to "crawl" to fill the right tangles in the process of thermosetting. If we don’t lengthen the stencil openings, the solder paste can melt only at the bottom of the IC, and the IC sides are unlikely to get sufficient tin.

Details of Tip 5 -

When PCBONLINE's engineers design SMT stencils, the width of the openings is to allow at least the 4 largest solder balls to pass smoothly. The purpose is also to prevent solder ball residue.

The Effects of SMT Stencils on PCB Assembly

From the above tips that PCBONLINE offers, you can understand that designing SMT stencils is not as easy as a piece of cake. Suitable PCB stencils make the PCB assembly:

  • Have sufficient solder for circuit connection;
  • Be void of assembly problems such as poor welding and insufficient solder;
  • Reduce the chances of repairing and extra testings so as to save costs and time;
  • Enhance the process's effectiveness and ensure product quality.

On the contrary, if the SMT stencils are designed without considerations to stress concentration, solder ball residue, and so on, the less suitable PCB stencils make the PCB assembly of lower quality, lower effectiveness, and higher costs due to repairing and testings from time to time.

PCBONLINE - One-Stop PCB Manufacturer with Superior Quality

PCBONLINE provides PCB manufacturing and assembly for medical, automotive, industrial, aerospace, defense, communications, and consumer electronics. If you order PCBs or PCBA from PCBONLINE, you will also receive the above-mentioned suitable PCB stencils for assembly. Why? Because PCBONLINE has leading capabilities and 22 years of experience in solving all possible PCB/stencil/assembly/component problems. Here are the reasons to choose PCBONLINE as your one-stop PCB manufacturer:

SMT assembly

  • PCBONLINE has three manufacturing bases for PCB fabrication and assembly;
  • ISO 9001, IATF 16949, REACH, UL, RoHS, and IPC-A-600/IPC-A-610 certified;
  • All levels of production happen on the same lines and have the same high quality;
  • Free one-on-one engineering support (DFX);
  • Free complete PCBA sample and functional tests for orders with an amount at or above $5,000.

Conclusion

The above SMT stencils designing tips are original by PCBONLINE concluded from many years of designing and manufacturing experience. If you are designing your own PCB stencil, take these tips for your design. If you are looking for PCB manufacturing and assembly, choose PCBONLINE as a priority.