Q: How does the cost of Thermal Clad compare to those of FR4?
A: The simple answer is: it does not! To make a true and useful cost comparison, one should compare the cost of FR4 + heat sink + device mounting hardware (such as clip, screw or clamp) + interface (Sil-Pad) plus the cost of manual assembly versus the Thermal Clad automated surface mount assembly. Taking into consideration all relevant costs, Thermal Clad system is lower overall when comparing it to a traditional printed circuit board (PCB) and heat sink assembly.
Q: How does the cost of Thermal Clad compare to those of DBC?
A: Again the costs have to be like for like. Unlike Thermal Clad, Direct Bonded Copper (DBC) always involves some method of mechanically attaching it to a heat sink. This is either a solder operation or a mechanical frame to secure the DBC. It is not possible to secure the DBC with fixing holes built into the delicate ceramic whereas Thermal Clad can support fixing holes, rivets, pressed studs etc. In simple terms: Thermal Clad can be worked like any metal sheet, even forming into a box-shape is possible. Taking into consideration all relevant costs, Thermal Clad system provides lower overall costs when comparing it to a DBC substrate and heat sink assembly.
Q: Our company does not do surface mounting. Can we still use Thermal Clad?
A: Yes, you can! Even if your company does not have the capability to assembly Thermal Clad, it is still possible to take advantage of Thermal Clad's high thermal performance and also remove the need for a number of clips and screws per power device. Thermal Clad can be used to solder all of your power devices in a row and create a sub-assembly. The device leads are formed, then "through hole" mounted into your control PCB. Bergquist can offer names of sub-contract manufacturing assembly houses to build these assemblies. We call this system: Thermal Clad-Power Rail.
Q: Do I need an interface between Thermal Clad and any additional heat sink?
A: This depends on your application. For example in thermally demanding applications you will be concerned about transferring as much heat to the heat sink as possible. By assuming intermat contact between Thermal Clad and the heat sink you create the best possible heat path. Bergquist have a material designed to interface Thermal Clad to heat sinks called Hi-Flow 225FAC which is applied as a pad. It softens to a paste like consistency and wets the surfaces when it reaches a threshold temperature. In addition to this Thermal Clad can be formed with a bias so the part ends up convex. When bolted down with fixing holes around the edge, the part is assured intermat contact.
Q: Why do you offer copper based as well as aluminium based Thermal Clad?
A: There is a need for both. Depending on your application, either copper or aluminium can be the best choice, since they both have their own specific advantages. Aluminium is a good choice for cost and thermal properties. But if your application demands heavy copper weight traces or tracks due to large currents or for mounting ceramic devices, then, as the copper thickness approaches 10% of the thickness of aluminium, the base material should be changed to copper. The use of a copper base for Thermal Clad is especially suitable when using naked devices or ceramic chip capacitors. Copper is closer to these components in thermal co-efficient of expansion which makes for more reliable solder joints. If you carefully select a thinner copper base versus the thicker aluminium base then the costs can be similar.
Q: What circuit finishes are available?
A: Any type of finish that can be applied to regular FR4 can be applied to Thermal Clad as well. This traditionally ranges from Hot Air Solder Levelling (HASL) for the reflow process and nickel-gold for Al wire bonding as an example. A solder mask is often used to keep parts clean and help with pattern recognition, it also contains the moulten solder during the re-flow process.
Q: Do I need to fix my power devices in place during re-flow to stop them moving around?
A: This is not required. In most cases if you design the solder pad area to mirror the exposed copper area of the back side of the device, the surface tension created in the molten solder during the re-flow process will bring badly aligned parts into the correct position.
Q: I would like to have some parts to test. How do I go about getting prototypes?
A: You simply contact us. Bergquist can take care of all of your prototype needs. Obviously we can supply them ourselves, but we also have a network of local printed circuit shops familiar with processing Thermal Clad. They can supply prototypes on our behalf or you can work directly with them.
All you need to get started is:
• Circuit pattern layout (preferably in electronic format i.e. Gerber)
• Mechanical design giving:
- positions for fixing holes and their size
- overall dimensions and
- importantly a dimension linking the artwork traces to a mechanical feature.
(i.e. dimensions of fidusial to datum edges)
• The material specification
• The required finish will complete the information we need
Q: What are the basic sizes of Thermal Clad?
A: The basic sizes ar: 609 x 457mm (24 x 18 inches) or 482 x 406mm (19 x 16 inches). Each panel has a border of 3.2mm (0.125 inches) which should be kept clear of active electrical tracks / traces. The border around the outside of the panel is normally used for tooling holes during the circuit processing of the material.
Q: Which is the lowest cost method for extracting my part out of the panel?
A: There is no simple answer to this one as each customer design will be different. As a general rule if your volumes are going to go above 5000 pieces then you might want to consider a punch tool. If the part has an irregular shape then punching or routing is normally used. If you design a rectangular part (i.e. no radii) then v-scoring offers good material utilisation.