-
- News
- Books
Featured Books
- design007 Magazine
Latest Issues
Current IssueOpportunities and Challenges
In this issue, our expert contributors discuss the many opportunities and challenges in the PCB design community, and what can be done to grow the numbers of PCB designers—and design instructors.
Embedded Design Techniques
Our expert contributors provide the knowledge this month that designers need to be aware of to make intelligent, educated decisions about embedded design. Many design and manufacturing hurdles can trip up designers who are new to this technology.
Manufacturing Know-how
For this issue, we asked our expert contributors to share their thoughts on the absolute “must-know” aspects of fab, assembly and test that all designers should understand. In the end, we’re all in this together.
- Articles
- Columns
Search Console
- Links
- Events
||| MENU - design007 Magazine
Bleeding Edge: A New Thermal Printed Circuit Technology: Thermal Plane
February 2, 2007 |Estimated reading time: 4 minutes
After many years of experimentation, the R&D department at Sierra Proto Express has invented a new method of printed circuit heat control. In place of hard-to-manufacture drilled metal cores, Sierra offers "Thermal Plane."
As the number of drill-out clearance holes in the metal core increases with square area, the ability to drill the thick copper/invar or aluminum metal core diminishes. The holes break into each other, allowing large sections of the metal core to be removed. As the number and area of the drill-out sections increase, the metal core's ability to transmit the thermal conductance of the package decreases. On some designs, the drill loss is a significant amount of copper or aluminum volume. As the drill-out area increases, the number of possible shorts from stringers increases. When you drill two holes close to each other, the radius creates short, fine stringers. They are very fragile and move out of position easily, creating shorts.
<?xml:namespace prefix = w />
An actual metal core used in thermal heat-sinking PCB.
You can see in the picture how the stringers can short and drill-outs reduce the area of the heat-sinking material; this is especially true inside chip packages, where the core material is needed the most.
To solve the problem and improve the thermal conductance of core technology boards, Sierra's R&D department developed a new method to manufacture heavy copper thermal planes. As we have inner layers comprised of power planes and ground planes, why not use them for multiple jobs?--first, as the power and ground circuits they were designed for, and second to remove heat. If we manufacture the power and ground planes with 4 to 8 ounces of copper, they will act just like a metal core but with improved thermal and electrical performance.
The power and ground heavy copper planes also offer benefits other than improved heat removal. The heavy copper planes reduce voltage drop noise created by resistance. As well, the large thick copper planes improve current distribution. The same effective copper thickness as an aluminum metal core can be built directly as the power and ground planes. Aluminum conducts heat at 160 w/m*C. Copper has 2.5 times the conductivity at 401 w/m*C; 8 oz. of copper is 12 mils thick, which is the same thermally as 30 mils of 6061 t6 aluminum. To have 8 oz .of copper in the center of a board, the power and ground plane are both made from 4 oz. of copper. The heavy copper planes can be placed further up the stack, closer to the top where the heat is generated. A 4 oz. copper plane one layer down from the chip is many times more conductive to heat removal and spreading than an aluminum core in the center of the printed circuit board.
When we laminate a drilled aluminum core in the center of a multilayer, several problems are created. First, the aluminum core has a significantly higher coefficient of expansion compared to copper. This expansion creates alignment problems for drilling. To compensate for the increased movement and subsequent drill alignment problem, the aluminum core initially is drilled as large as possible. The result is reduced heat-sinking volume from the increased hole size and increased cut-out areas. The metal-core PCB also suffers from a filling problem: The aluminum core is 30 mils thick, which is too thick to fill with prepreg alone. The additional epoxy filling process is not without significant problems. The filling material is expensive and difficult to use. The filling material also suffers from a high shrinkage ratio. Some other problems have been encountered with poor electroless adhesion to the filling materials. The thinner 4-oz. copper power plane thermal solution is easily manufactured with normal prepreg, thereby eliminating the problematic extra filling process. The heavy copper thermal plane can be formed to fine features, typically seen in the power and ground image.
Normal copper plating techniques are limited to 4 mils per oz. of line width and resolution. Through R&D, we found a method of manufacturing the 4- to 6-oz. power and ground planes to 2-3 mils per oz. of copper, allowing the designer a minimum of 8- to 18-mil line width and space resolution for a 4- to 6-oz. thermal plane. Because of the finer resolution of plating the thermal plane, we do not lose the very important heat-sink copper volume due to drilling.
The power and ground Thermal Plane can also be constructed with thin prepreg to improve inner-layer capacitance, lowering power supply and return noise.
The typical center-located aluminum/copper/invar metal core is the furthest possible distance from the generating chip heat, which means lower overall thermal dissipation. Heat transfer rate is the square root of the distance. There are no direct thermal connections to chips or heat-generating components with metal cores.
The thermal vias in the power and ground plane significantly increase overall thermal conductance as they are mechanically connected directly to a pin on a heat-generating chip. Fiberglass has a very low .24 W/m *C conductivity compared to copper's 401 W/m *C.
Even a few direct connections to the heat-generating chips will result in significantly more heat conducted than through low-conductance fiberglass to the spreader power metal cores. Each chip will typically have at least two power and ground connections.
The Thermal Plane technology is thinner (about 21 mils on a 6-layer two-plane design) because the old power and ground planes are now doing double-duty as electrical distribution layers and thermal distribution layers.
Power layer = 1.5 mils Thermal Plane power layer 6 mils
Ground layer = 1.5 mils Thermal Plane ground layer 6 mils
Dielectric layer 2X 5 mils Dielectric layer 2 X 5 mils
Metal core AL 30 mils
---------------------------- ------------------------
Total 43.0 mils 22.0 mils
Savings 21 mils
The ability of Thermal Plane technology to reduce the cost of the printed circuit board, improve the heat removal and significantly increase the temperature spreading capability, can improve many existing metal core boards.
Authors: Robert Tarzwell, Directory of Technology, Sierra Proto Express; Ken Bahl, CEO, Sierra Proto Express