PCBs have many electronic components that are almost exclusively soldered with an automated process. There are other components that can also be soldered onto a PCB, and these are not always electronics. Heatsinks and metal shielding cans are two other types of components that are sometimes mounted onto a PCB using solder. In a low volume process, these will normally be hand soldered, but these components must be placed and soldered with an automated process in moderate to high volume production.
In order to bring these components into an automated process, wave soldering or reflow soldering would typically be attempted, but the process will not proceed properly without the right processing parameters. If you do plan to use mechanical elements that will solder onto a PCB, make sure you understand these potential problems in the soldering process. We’ll also show some examples of large mechanical components that can be soldered onto a PCB.
Rules For Solderable Shields and Heatsinks
The two most common types of mechanical components that can be soldered directly onto exposed copper on a PCB are heatsinks and EMI shielding cans. Both are typically placed as through-hole components, and both types of components are typically built from copper, a copper alloy, aluminum, or pre-tinned steel. The challenge in working with these components is in picking the soldering method, overcoming the large thermal mass of these components, and wetting of solder onto the metal.
The proper processing steps and materials needed to solder these components include:
- If required (such as with aluminum), pre-roughen the soldering surface through a gel flux
- Use a specifically developed for that metal (special pastes are available for aluminum)
- Sufficiently high soldering temperature and soak time (for reflow)
- Fixturing can help hold these components in place during soldering
EMI Shielding Cans
One of the most common board-mount components that would need these more specialized soldering techniques are shielding cans. These components are large and have high thermal mass compared to other components, and they are often mounted as through-hole components. The larger thermal mass might require longer soak time or higher temperature in reflow, and depending on the material used to build the can, a special flux/paste might be needed.
These shielding can outlines can help prevent noise coupling between different regions of the board and allow a shielded enclosure to contact a PCB.
If special flux and/or paste is needed, these cannot be placed into a reflow process alongside other solders, or selective dispensing of this alternative paste would be needed for the mounting points on the shielding can. Finally, depending on the shape and size of the shielding can, fixation might be needed if the entire assembly will pass through SMT. In some cases, selective soldering might be used to attach just the EMI shielding can.
Through-hole/SMD Attach Heatsinks
Heat sinks that are bonded directly to components are typically attached with a thermal interface material, such as a thermal paste, thermal pad, or even a conductive silicone. Board-level heatsinks can also be mounted directly to the PCB, such as in the example below. These can be either SMD or through-hole components with weld tabs.
This type of mounting can be difficult due to the soldering limitations mentioned above. However, it is best from an EMI standpoint and thermal standpoint. It is also has advantages in instances where heat needs to be pulled through a surface of the board, rather than through a component. In this final use case, the cost of comparable thermal compounds and their dispensing process could add more cost to assembly.
TO-220 and DPAK Heatsinks
One heatsink variation that can also be placed into standard soldering processes are metal heatsinks specifically designed for certain integrated circuit packages. These heatsinks are designed to conform to the component package and they will often include fins that can sink heat into a flowing airstream. An example for a TO-220 component package (such as a power MOSFET) is shown below.
Example stamped anodized heatsink for a TO-220 component. This particular heatsink component could be screwed onto exposed metal, bonded with a TIM, or soldered between a die-attached pad and exposed copper on a PCB.
The process for working with this type of component could involve two reflow steps, or a reflow step followed by selective soldering:
- The heatsink is placed and soldered to the PCB in a reflow process
- The TO-220/DPAK component is placed in the board with specialized paste
- The heatsink is soldered to the TO-220/DPAK die-attached pad
Why would we use this kind of heatsink with soldering directly to the exposed TO-220/DPAK pad? The reason is that we can provide a ground connection to the system or chassis ground for both the exposed pad and the heatsink. Soldering the pad directly to the heatsink and ground provides a low impedance path for any ESD that may occur near the heatsink. In some cases, the metal part that is being bonded to the heatsink does not accept a screw (see the screw hole above), so you have no choice but to solder and close the contact seam with a thermal compound.
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