Posts Tagged ‘SMT’

All about SMT Stencils

Written by Admin on . Posted in PCB, PCB Assembly and component

SMT

What are Stencils?

Surface-mount technology (SMT) is a method of producing PCBs where the components are mounted or placed directly onto the surface of the board.

The stencil is designed to allow a smooth transfer of material (solder paste) onto the bare PCB. It is crucial in ensuring that the material is placed with precision. It is also vital in ensuring that deposits are formed with the proper shape and size. The stencil is critical to ensure that the assembly process can be operated with a high-yield output.

The stencil provides a tool for accurate solder paste deposition, allowing the process to be repeated multiple times. The solder paste is printed through the holes in the stencil, forming deposits that hold the components in place. The stencil provides the accuracy needed to ensure the solder is printed in precisely the right place on the PCB.

The way in which the stencil is designed can be varied based on thickness, the size of the aperture, and the shape of the aperture.

How are stencils formed?

There are various technologies for forming stencils

  • Laser
  • Electroformed
  • Chemically etched plastic
  • Hybrid – a combination of chemically etched and laser-cut.

In chemically etched stencils, the stencil is etched from both sides using two positive images. The etching process leaves aperture walls that taper to an hour-glass shape in the centre of the aperture. In laser-cut stencils, a stainless steel foil is cut by laser, creating an opening for each component that will be included in the final PCB. For both processes, aperture walls are electro-polished to ensure a smooth finish. Following the cutting process, the stencil is aligned over the board. The solder paste is deposited over the apertures. After the solder is laid, the foil is removed.

Ensuring the correct amount of solder paste

It is vital to ensure that the amount of solder being laid down meets design specifications.  If not enough solder is used, there is a risk of inadequate solder joints. In contrast, much solder can result in balling, bridging and tomb-stoning. Both options can disrupt the electrical functionality.

Types of Stencils

There are four types of stencils:  prototypes, framed, frameless and hand/ rework stencils. Each has a different function.

  • Prototype stencils are customised laser cut stencils made for various PCB needs. They are also used in Gerber files. They are generally designed for manual printing.
  • Framed stencils (also known as glue-in stencils) are laser-cut stencils mounted permanently on a stencil frame. They are designed for high speed printing on PCB, and for production runs.
  • Frameless stencils do not require permanent fixing to a frame. They are mainly used for screening printing on PCBs, and are a useful option for short-run production and PCB prototypes.
  • Hand/ rework stencils are used to print individual components onto a PCB. In general, they are used in rework situations, but can also be used in developing prototypes. The stencil includes a flag that allows the stencil to be hand-held.

Mirror Mirror

Written by Admin on . Posted in PCB Assembly and component

A mirror can bring bad luck, it is said. In this PCB assembly challenge, it certainly did when a mirrored pad layout for a transformer made it impossible to mount the component to its intended location on the top side of a PCB in its usual orientation.

Design error: A mirrored pad layout creates orientation problems between pads and component pins; layout is for bottom-side rather than top-side mounting.

Design error: A mirrored pad layout creates orientation problems between pads and component pins; layout is for bottom-side rather than top-side mounting.

 

The component’s footprint, it turns out, would work fine if it were on the opposite side of the PCB, but that bottom-side installation is not possible.

Flipped upside down, the SMT transformer’s pins line up fine, except that they are facing upwards. But we can still mount the component and make a robust connection using adhesive and connecting wires.

Flipped upside down, the SMT transformer’s pins line up fine, except that they are facing upwards. But we can still mount the component and make a robust connection using adhesive and connecting wires.

 

The customer made a design mistake; although the pads for top-side SMT mounting of the component are in place, they are in mirror-image orientation; e.g., the pad layout with Pin 1 is intended to be installed from the bottom of the board. Consequently, it doesn’t match up in terms of orientation on the top side of the PCB unless the component is literally placed onto its back. But that means that the leads are sticking up into the air, pointing in the wrong direction.

 

Small dots of epoxy are applied to the PCB surface and to the component body, before it is attached, the epoxy cured, and the transformer connected pin by pin.

Small dots of epoxy are applied to the PCB surface and to the component body, before it is attached, the epoxy cured, and the transformer connected pin by pin.

 

It’s well known that a dab of epoxy can cure a host of ills, and in this case it was simply a matter of dispensing a tiny amount of epoxy onto the back of the component body, in the center, as well as onto its intended location on the SMT PCB assembly.

The component is then carefully located in place upside-down and the epoxy cured. With the component robustly mounted in this manner, small wires were then run from each lead (pin) to its corresponding pad on the board’s surface.

 

It requires skillful hand soldering once the component is in place, but the connection is robust and complete.

It requires skillful hand soldering once the component is in place, but the connection is robust and complete.

Wicked Wicking

Written by Admin on . Posted in PCB Assembly and component

 

This PCB assembly challenge involved attaching a solar panel to one side of a pad using solder paste with a pass through an SMT reflow soldering oven.

Solder wicking 1

Solder wicking through the unmasked vias to the back side forms unacceptable “bumps” on top of the vias

The attachment or bond itself wasn’t the issue; but after the first trial runs, it was clear that solder wicking through the unmasked vias was going to be. Solder would wick through the unmasked vias to the back side and form “bumps” on top of the vias.

These bumps made the surface nonplanar and of course were unacceptable. It wasn’t an issue of using excess solder paste. But the “wicked wicking” had to be stopped, or at least prevented.

But how? Clearly, to keep the solder where we wanted it to remain during reflow, we had to find a way to prevent it from wicking up, collecting at the opposite ends of the vias and forming bumps. We had to find a solution that was simple, temporary, and tolerant of reflow soldering temperatures. The answer was Kapton polyimide tape, a familiar product to PCB assemblers for many years, and a material that does not degrade at reflow temperatures.

Kapton tape - molten solde

Kapton tape is applied to cover the unmasked vias; it will block the molten solder from leaking through.

Kapton tape was applied to cover the unmasked vias in order to block the molten solder from leaking through the vias to the back side during reflow. After reflow and cooling, it was a simple matter to peel off the tape. This temporary masking solution worked; there were no more solder bumps on the back side of the assembly, and the cost of the fix in terms of time and material was very low.

temporary masking solution PCB

Figure 3. This temporary masking solution worked; there are no more solder bumps on the back side of the assembly.