Introduction
As the world is growing faster in technology advancement, the need for resilient, robust and optimized design is also emerging. Designers are working on the techniques to increase efficiency and reduce size and cost of product.The integral part on any electronic device or product is its PCB. The PCB is made up of various materials among which the most popular is FR4. The PCB has traces, tracks, layers, vias, and holes and solder mask to give the product its physical shape.There are many constraints in PCB in which the most critical is the PCB Temperature.
Content
1.PCB Temperature
2.PCB Soldering Temperature
3.High Temperature PCB Material
1.PCB Temperature
The temperature of PCB is directly related to the amount of heat generated by the components soldered on the PCB. The through hole components are most widely used components in many practical applications like inverters, UPS, Rectifiers, SMPS, Linear power supplies, Computer power supplies, Motor Drives etc.
1.1 Types of Components Generating Heat
The amount of heat generated by a component is proportional to the load current flowing through it. In case of MOSFET for example, the drain current I(D)is the load current and it determines the heat generated by MOSFET. Since the MOSFET Drain to source resistance RDS(ON) is very small so it can cause little resistance and drops very negligible voltage across it hence little power is dissipated.
Through Hole
There are many components like IGBTs, MOSFETS, TRIACS, and SCR, HEXFETs that generate heat and can cause the PCB to heat at very considerable rate. These components are generally through-hole components and require big heat sinks to dissipate heat in air. As we can see in figure 1 this N-Channel MOSFET has metallic heat sink on back which will be soldered over PCB to dissipate heat
SMD Components
As we can see in figure 2, this is the TO-252 DPAK package, N channel power MOSFET. This also has the metallic tab that will be connected to GND to dissipate heat in air. The metallic tab is generally the Drain (D) of MOSFET so that it can carry heavy current of 20A continuous through it. This heavy current will surely generated heat that will be dissipated via tab. Its junction temperature is 150 ℃ and power dissipation of 20 Watt.
1.2 Methods to Reduce PCB Temperature
The most common method to reduce the heat generated by high power devices is to use heat sinks. There are various types of heat sinks available of various size and shapes.
Heat Sinks
The heat sink is used to transfer thermal energy generated by an electronic assembly or component to a cooling medium. Heat is always transferred from a high temperature body to a low temperature body (fluid medium) by conduction, convection, radiation or by the combination of any of these heat transferring methods.
Cooling Fans
Cooling Fans are usually used with the heat sinks to further reduce processor temperature. These fans provide duct for hot air to flow out and cool air to flow in from surrounding.
2.PCB Soldering Temperature
The PCB fabrication material like copper, bronze and silver are very good to be soldered and creates a strong physical connection with soldering wire material. The soldering wire material tin and lead are used widely. There are some material like titanium and aluminum which are impossible to solder. This is fortunately good because these materials are used in the tip of soldering iron and soldering machines.
Pure tins melts at 203 ℃ while lead melts at 330℃. The mixture / alloy of the two (tin and lead) in the ratio of 60/40 melt at 183 ℃. The tip of the soldering iron should have the temperature of 600F or 315℃ enough to easily solder SMD parts.
3.High Temperature PCB Material
When it comes to the robustness of PCB material during soldering process, the temperature is very important to consider. The FR-4 material is commonly used PCB material with glass temperature Tg = 135℃.
Tg is the measurement of temperature at which the PCB will begin to deform. If the operating temperature of PCB is greater than the Tg then the PCB will begin to melt or change shape which pose adverse effects
Hence the Tg values are always greater than the operating temperature of PCB. The high temperature PCB which are more resilient towards heat are mentioned
Material
|
Tg (DSC, °C)
|
Td (Wt, °C)
|
S1141 (FR4)
|
175
|
300
|
S1000-2M (FR4)
|
180
|
345
|
IT180
|
180
|
345
|
Rogers 4350B
|
280
|
390
|
Where Td = decomposition temperature
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