Careful Design Steps Critical for LED PCBs
Although LEDs are steadily growing more popular in many applications, in recent years that growth has greatly and dramatically accelerated. This is due to the explosion of smartphones, iPods, and tablets, as well as other similar small and portable products for consumer, industrial, military, aerospace, and commercial markets. The ever shrinking liquid crystal display (LCD) displays used in these products now require white backlight versus the previously used standard green light. LEDs are quickly becoming the light of choice due to their shrinking size, as well as declining cost and longer life.
High Voltage Drop
On the other hand, LEDs pose a host of issues to the PCB layout designer, including thermal management challenges. The most prevalent is the fact that LEDs have a high voltage drop. Depending on the chipmaker, this can run between 3.1 to 4 volts, which is substantially large compared to the normal 1.08 or 2.7 vaults. Hence, the PCB designer is looking at a huge voltage drop and they wonder how to incorporate it in his or her design.
To maintain the white backlight compared to the earlier green light, battery voltage has to be increased. By increasing battery voltage, the size of the switching regulators is also increased, thus increasing the amount of needed circuitry. There are two ways of doing that; with the integrated LED circuitry, it can be placed close to the LED or separately.
The common practice today is to separate driver circuitry from the LED assembly to avoid the heat transfer from the LED assembly to the driver circuitry.
There are several key PCB design considerations involved here, but thermal management is foremost among them. Within thermal management, there are a few other key issues to ponder; those involved PCB material, thermal vias versus copper plate, and junction temperature.
As far as PCB material, there is the traditional, cost-effective, widely available FR4 versus a metal core (MC) PCB. The biggest issue with FR4 is its poor thermal conductivity. On the other hand, a PCPCB, as shown in Fig. 1, is great thermal conductor, but it costs 20 to 30 percent more than FR4.
Take for example a very bright power LED that is unable to use FR4 material. First, this particular LED and its level of power energy generate considerable heat. Second, FR4 doesn’t provide a direct thermal path from the junction of the LED’s base to the ambient. It has to use thermal vias or add copper plate. When copper plate is added, there is a slight chance it might short the LEDs if not connected properly. By using thermal vias, you are limiting the number of fab shops capable of making perfect thermal vias. In cases like these, the MCPCB is used because it effectively spreads the heat without encountering issues that the FR4 otherwise does.
A more reliable technique for designing such an LED system is to design with it a controlled temperature rise for a given power dissipation. A way to do that is to use a controlling temperature resistor using a high thermal conductivity heat spreading material. Hence, an MCPCB and natural graphite, which is a heat spreader, are normally used for power LEDs. These two are proven to be thermally conductive materials that properly manage thermal flow and maintain temperature uniformity within the LED system.
The third issue associated with thermal management is junction temperature, which is at the base of the LED. It’s highly important that it be properly measured. If it’s not, that junction temperature won’t be effectively dissipated. Moreover, if thermal management isn’t properly performed, then over time, poor thermal management will degrade the color at the output of an LED. Color and consistency of brightness will change, plus the LED will not last as long as it has been designed for.
Another key consideration deals with coefficient of thermal expansion or CTE. There are thermal characteristics and impedance mismatches associated with various materials. It is critical that all materials being used are as closely CTE matched as possible. The reason is if an extremely high thermally conductive material comes in contact with a very low thermal conductive one, board de-lamination will result, as shown in Fig. 2.
In addition to proper PCB designs and without question, OEMs producing smartphones, tablets, and other small and portable products want assurances of longer life for those LED-based backlights beyond the longer life specification LED suppliers promise on their datasheets. This is where PCB design expertise about thermal management and related issues kicks in to assure and extend those longer life specs that LED suppliers tout. This is especially valuable to OEMs producing those newer portable communications devices since the lifespan of their products is determined by the lifespan of those LEDs.
So here, the common denominator in all LED-based PCBs is the importance of proper heat dissipation. For starters, heat that is not dissipated causes the P-N junction of the semiconductor needs to rise above the safe level, thus making the LED considerably more prone to failure. As a general rule of thumb that experienced LED-based PCB designers follow is in electronics systems, reliability and longevity are highly dependent on temperature. Temperature reductions as small as >5°C (>41°F) can double the service life of an electric device.
An associated design rule is to maintain a correct power supply to avoid overpowering LEDs. If an LED is overpowered, the temperature of the semiconductor layers rise above the safe level, thus the tendency for the device is to fail or malfunction and the lifespan of the device deteriorates.
Also, it’s important to note that there is a variety of LED packaging for different types of LEDs, but here, the focus is on LEDs mainly used as backlight or status indicator light for multi-functional and thin profile portable applications such as smartphones, tablets, and other similar small products.
Like in all other LED board applications, the heat sink plays a prominent role in thermal management. The PCBB designer must select the correct heat sink for this small application. In cases like this, the proper heat sink may be a vertical heat sink with fins.
Equally as important as designing in the right heat sink, the PCB designer must also connect that heat sink to a thermal pad, using the thermal grease or compound as shown in . The thermal pad provides a path from the LED to the bare board and then to the heat sink. LED chip temperature is thus decreased and the device’s lifespan is extended. In some instances, inexperienced PCB designers aren’t aware of the importance of this thermal pad. Heat generated from such small packages, for example as small as 5/16-inch diameter, becomes a concern due to the increasing amount of current applied in such a miniscule area. Without a way to exhaust the heat – meaning if a thermal pad were not used, the life of the part would surely be short lived.
Via In Pad
Via in pad technology is yet another tool the PCB designer can use to better manage the heat in an LED board design. It allows designers to utilize space more effectively when board real estate is critical. Typically, via in pad involves drilling a hole in the center of an SMT pad, filling it with a conductive or non-conductive material, and then plating over it to leave a flat surface so the hole is not visible.
This allows the SMT part to be placed directly on top of the hole. If the hole or via is left as is and not filled, solder would wick through the vias during the assembly process and starve the pad of a solid, reliable connection. Via in pad is particularly suited for in LED board applications because it provides a highly conductive path for heat dissipation from the heart of the LEDs to cooler regions of the board, like the power and ground planes.
When it comes to designing via in pad technology in an LED board, the following steps are important for the PCB designer to follow; via hole size should be small, around 6-8 mils. Vias should be placed on a 25-mil grid and they should be electrically connected to one of the copper planes on the internal layers and preferably on the bottom layer of the board, as well. Fab notes should properly identify the via types that will be filled and plated during manufacturing.
Unfortunately, in some cases, naïve, inexperienced PCB designers either fail to realize the importance of via in pad for small densely populated boards like LED PCBs and don’t use it or they use via in pad incorrectly creating unnecessary issues. Those issues include not using the correct via size; the correct amount being 6 to 8 mil vias. In other instances, the PCB designer designs only a few vias.
Here, those very few vias have limited thermal connectivity so they should be used generously. Also, vias are sometimes not connected to large, low resistive copper planes. Lastly, vias can be improperly identified in a fab drawing and can be missed during the filling and plating process. This may result in some or all vias being open, causing soldering issues during assembly such as solder wicking and shorts.