Engineers must utilize tiny SMT components onto circuit boards (PCBs) because electronic equipment is becoming thinner and smaller. The board often has a mixture of medium, fine pitch, as well as huge SMT components. Our company suggests using step stencil SMT for these boards’ construction in order to achieve success.
What Does a Stencil Offer?
Solder paste must be applied to PCB pads before SMT components may be assembled on the circuit board. Also, the best and most precise way to achieve this is via a stencil. The stencil can be described as a thin steel sheet with precisely matched holes that correspond to pads on your PCB.
The operator can squeeze the solder paste via the stencil’s holes as well as deposit them onto the PCB’s pads after lining up the stencils on your circuit board. The operator applies solder paste on the stencil using a squeegee, spreading it out evenly.
Some of the solder paste is still on these pads when the operator takes the stencil out of the board. Whenever your pick-and-place machine receives the board having solder paste, it places the proper SMT components upon that solder paste. Also, the boards that have been populated are then soldered on in reflow furnaces.
Difficulties with stencils
The amount of solder paste which the stencil usually deposits onto the pad depends on the stencil’s thickness and the size of the aperture or opening for the pad.
For a component to be soldered properly, the amount of the solder paste present on the pad is crucial. The component’s soldering may not be sufficient if indeed the volume becomes low. Excessive solder might create bridging with nearby pads when the volume becomes significant. For optimal solder deposit, our company designs step stencils SMT with the right thickness and aperture.
A board containing a mixture of different SMT components containing various footprints might not have been suitable for the stencil, even if it has the ideal aperture and thickness. For instance, since this ceramic body helps in absorbing much heat and keeps the solder balls against melting at typical reflow temperatures, ceramic BGAs require a high melting temperature during the process of soldering.
For this, larger amounts of solder paste must be printed on its CBGA pads, necessitating a stencil with a thickness between 0.18 and 0.2 mm.
However, extremely tiny apertures present on your stencil are required for other types of SMT components, such as the 0402 chip component as well as QFPs having at least 0.5 mm in pitch. The apertures are better suited for stencils that have a thickness of 0.127 mm because using them on a stencil of 0.2 mm thick causes problems with the paste release whenever the operator raises the stencil.
On the board having QFP, CBGA, as well as 0402 SMT components, a step stencil SMT, which is a mixture of a fundamental stencil having a thickness of 0.127 mm having sections of 0.2 mm thick for the CBGA—is necessary to properly deposit solder paste.
What Does Step Stencil Mean?
For both big and tiny SMT components, the step stencil SMT effectively controls the solder paste volume deposit on your PCB. A broad stencil and more solder paste are needed for successful soldering of huge SMT components. Also, thin stencils work better for smaller SMT components because they don’t require as much solder.
Step Stencil Types
Step-down and step-up stencils are the two most popular varieties.
Step-up stencils are used on PCBs that have a lot of small components including a few big SMT components. The steel sheet measuring 0.1 millimeters thick serves as the stencil’s fundamental material; big SMT components require steel sheet measuring 0.15 mm thick. On its basic sheet, the larger thickness creates a convex layer that stays flat on that side facing your circuit board.
Step-down stencils are used on PCBs that have a lot of large components as well as a few tiny SMT components. Steel sheet with a thickness of 0.15 mm serves as the fundamental stencil for smaller SMT components; this thickness is reduced to 0.1 mm. On the sheet, the reduced thickness creates a concave layer while remaining flat on that side that faces the PCB.
Step stencils can be used without or with frames. For different production runs, assemblers employ step stencils that are framed; however, step stencils that are frameless are better suited for the prototype assembly. Step stencils without frames are cheaper compared to the step stencil smt with frames.
Fabrication of Step Stencils
Three different technologies are employed by manufacturers to create step stencils. They apply:
Technology for Photo-Chemical Etching
This serves as the main technique for the fabrication and creation of step stencils. The fabricator uses a photographic procedure to impart resist to the stencil’s surfaces that won’t be etched. After that, they use an etchant to take out the material from the stencil’s surface until the appropriate thickness is reached.
This same fabricator affixes the fundamental stencil sheet onto a vacuum plate that has been cooled and then on a milling machine. They remove relatively little quantities of material using the milling machine (CNC based) until the appropriate thickness is reached. They fix each stepped foil to the frame as well as cut these apertures using the laser beam.
A thin sheet, as well as a much thicker sheet is both used by the fabricator. They remove the step’s area from the thicker sheet and main thin sheet using CNC machines.
Advantages of step stencil SMT
Step stencils have a number of advantages, such as the
- Ability to control the volume of the solder paste optimally,
- Eliminate numerous print processes, replace single stencils
- Allow for the possibility of squeegee steps as well as relief on the same stencil’s board side.
Different volumes for the solder paste are needed for different SMT components on complex PCBs. While properly managing the deposit of the solder paste in the fine pitch sections, using different foil thicknesses within the stencil helps in depositing enough solder on the rough pitch areas.