How to make high power PCB adapt to high temperature environ

2019-12-16 13:58Writer: qyadminReading:
  As the commonest components integrated platform. multi-layer PCBs connect circuit boards and components together. With electronic products becoming light, thin and small in size. and having high performance, IC components have become integrated. leading to the high integrity of PCBs. As a result, heat production has increased and thermal density of PCBs has gone up especially. because of the mass use of high-frequency IC components such as A/D or D/A type and moving up of circuit frequency. If massive thermal loss fails to sent out. the reliability of electronic equipment will influenced.

      According to statistics, among the elements leading to the failure of electronic equipment. temperature accounts for as high as 55%, as the top cause. With the temperature increasing, the failure rate of electronic components will increase . Once the environment temperature increases by 10°C.  the failure rate of. some electronic components can increase to twice large. For aerospace products. this type of thermal control design can't. even ignored as the inappropriate design method. for the all kinds of circuits in special environment. will result in the complete failure of the whole system. Thus, much attention must paid to thermal design during PCB design.
  The analysis should begin or start with the cause analysis. The direct cause of high temperature of PCBs lie. in the existence of power consumption components. Each component has power consumption in different extent. that arouses the change of thermal strength. There exist 2 types of temperature increase phenomena: local temperature rising. or large area temperature rising and short-term temperature rising or long-term temperature rising. Heat transfer has 3 ways: heat conduction, heat convection and heat radiation. Radiation dissipates heat through electromagnetic wave motion passing through space. Since the radiation dissipation features a low amount of heat. it is usually regarded as an assisted dissipation method. This passage. will introduce a solution to PCB heat dissipation. in the process of long-term operation. in the environment with high temperature based on the heat conduction. and heat sink transient heat storage technology with a type of servo PCB as an example.
  On this servo PCB, there are 2 power amplifier chips with a power of 2W, 2 R/D conversion chips. 2 CPU chips, 1 EPLD chip and 1 A/D conversion chip. The power of this servo PCB is 9W. The servo PCB installed in an airtight environment with limited air convection. Besides, because of the limited space. cold plate dissipation can't installed on the servo PCB. to ensure the normal operation of servo PCB, only heat conduction. and heat sink transient heat storage technology can utilized to transfer the heat. produced from the PCB to the body.
  It is a common method to dissipate heat through metal core PCB. First, a metal board with excellent heat conduction embedded between a multi-layer PCB. Then, heat dissipated from metal board or disjunctive equipment. connected to the metal board to dissipate heat. The operating structure shown in Figure 1.


high power PCB


  The main material of metal core PCB covers aluminum, copper and steel. It can be also used as the ground layer. The upper layer and lower layer of metal core PCB. can  interconnected through plated through hole. and heat can transferred on the inner layer and surface of metal core PCB. Heating elements can soldered on the board. through the bottom and heat conduction hole. As a result, the heat generated by heating elements transferred to metal core PCB. that transmits the heat to the tangent chassis by the heat conduction hole and sends it out.

       PCBs with such a structure have a wide series of applications but they can also arouse some problems. Metal core PCBs are so thick that deformation tends to take place in the uneven heat dissipation. leading to the loose contact between chips on PCBs and pins. It's easy and quick for metal core PCBs to dissipate heat. which brings about enormous difficulties to chip changing. and in the process of chip changing; the local heat attraction. of metal core PCBs will lead to the serious deformation of PCBs. It verified that the larger area a PCB has, the more it deformed.

  to get the problems above solved, upgrading design must done to metal core PCBs:
  a. 4-layer copper foil with thickness of 0.15mm can nipped in PCBs. so that the thickness of PCBs can increase by 3mm to ensure that PCBs aren't deformed. and the through-hole reliability rises.
  b. About chips with heat generation of 2W, SMT pad can added to the bottom of chips. to transfer the heat to the metal layer of PCB.
  c. The chip bottom is capable of transferring heat to the internal copper foil layer. by the copper foil with a large area. and heat conduction through hole.
  d. The insulating layer on both sides of PCB can milled off to realize the PCB edge metallization. Heat dissipation can achieved through the contact between bare edge PCB and base. The installation can finished by 36 screws to increase the heat conduction of PCB and the body.
  After the implementation of the measures mentioned above. the upgraded PCB design shown in Figure 2.


high power PCB


to set up simulation modeling and analysis on servo PCB. the software FLoTHERM used for electronic equipment heat situations. The edge condition of servo PCB is: the environment is 65°C with the operating time of 90 minutes. The components on servo PCB all meet X derating need. The allowing body temperature of each component shown as the following table:
  ComponentsHeat Consumption/WMax Temperature of X Derating/°CMax Body. Temperature of X Derating/°C
  CPU Chip0.610087
  R/D Chip0.510087
  EPLD Chip0.510085
  Power Amplifier Chip2.010087
  The main power components on servo PCB include 2 chips (49.76mm*41.4mm) each of which has a heat consumption of 2W. The heat consumption of other components on servo PCB is 5W in all and the heat consumption of the whole PCB is 9W. servo driving components 10W, power supply 40W, and the heat consumption of servo. and power supply is 59W.
  The temperature of servo control chip shown in Figure 3.
high power PCB


  The heat analysis of operating for 90 minutes. in the 65°C environment shows: operating for continuous 30 minutes. the temperature of chip rises , reaching 72°C above. operating for continuous 50 minutes, the temperature of chip remains stable. operating for continuous 90 minutes; 

the body temperature of 2W chip (87°C) is 77.9°C; 
the body temperature of 0.6W chip (87°C) is 84°C. 
the body temperature of 0.5W chip (87°C) is 78.2°C; 
the body temperature of 0.5W chip (85°C) is 77°C;.
  Based on the calculation and simulation heat design operating condition. the servo control chip temperature remains in the reasonable range. Omit theoretical analysis, there's no space between chips and PCB by default. But in the actual process of installation, there is some space between them. and silica gel can used to fill the space to ensure the heat dissipation effect of PCB.

PCB temperature Dissipation design

       Radiation is the emission of energy in the form of electromagnetic waves. We tend to think of it as only things that glow.  but the fact is that any object with a temperature. above Absolute Zero radiates thermal energy. While it may be true that usually radiated heat. is the least influential in circuit board performance. sometimes it can be the straw that breaks the camel’s back. To remove heat, the electromagnetic waves should have a clear path away from the source.

        Reflective surfaces frustrate the exodus of photons. causing significant numbers of them to redound on their source. If by unhappy chance the reflective surfaces conspire to form a parabolic-mirror effect. they can concentrate the radiated energies of many sources . and focus it on one unlucky part of the system. causing real trouble. The key factors affecting thermal radiation. are the temperature of the source (in absolute units, raised to the fourth power). the thermal emissivity of the material involved. and the surface area available for radiation.
       Convection is the transfer of heat to a fluid – air, water, and so forth. Some convection is “natural”: the fluid absorbs heat from a source, becomes less dense. rises away from the source to a heat sink, cools, becomes more dense, sinks back to the source. and the process repeats. (Recall the grade school “rain cycle”) Other convection is “forced” by a fan or pump. The key factors affecting convection are the temperature difference between the source. and coolant, the ease with which the source transmits heat. the ease with which the coolant absorbs heat, the coolant’s flow rate. and the surface area over which the heat transferred. Liquids absorb heat much more than gasses.
      Conduction is the transfer of heat via direct contact between the heat source and the heat sink. In many ways it’s analogous to electrical current: the temperature difference. between the source and sink is akin to voltage, the amount of heat transferred per unit time is akin to amperage. and the ease with which heat flows through a thermal conductor. is akin to electrical conductance. In fact, the factors that make a good electrical conductor. tend to make good thermal conductors as well. because both represent forms of molecular or atomic motion. Copper and aluminum, for instance. are excellent conductors of heat and of electricity alike. Large conductor cross-sections improve conductivity for heat and electrons. And as is the case with electrical circuits, long. tortuous flow paths can degrade a conductor’s effectiveness.
       the primary mechanism for removing heat from a circuit board is to conduct it to a suitable heat sink. where convection bears it to the environment. Some heat wafted and radiated from the source. but usually the bulk drawn away. through designed channels called “heat vias” or “thermal vias.” PCB heat sinks are large, emissive. surfaces (often corrugated. or finned to further increase surface area) bonded to. conductive (e.g., copper or aluminum) backings. a labor-intensive process. PCB heat sinks also may connected to the unit’s chassis to avail of its surface areas. Often fans used to provide a flow of cooling air. which in extreme cases may itself cooled in a gas-liquid heat exchanger.
      Boiling this down. the heat-management options available to a designer are to reduce power densities, remove. or isolate the unit from heat sources, provide more powerful cooling mechanisms (e.g., bigger fans, liquid cooling systems, etc.). increase the size and accessibility of the heat sinks, use larger conductors. or use exotic materials capable of withstanding higher temperatures. All these have implications for the cost, size, and weight of the total system. that must e considered in the earliest concept-development and design stages.

 What is the best temperature for soldering circuit boards? 

      Temperature rise is something you have to consider. but usually the resistance and the resulting voltage drop. at full current have been the limiting factors. when I've gone through this. That said, 100°C is a large temperature rise. That's not enough to be a problem for a copper trace on a FR4 board by itself. but that's going to affect the clear ambient temperature for nearby components.
     If you have that much temperature rise. you're dissipating significant power in the trace. which means power loss in your system. Again, the first concern should be how much voltage drop you can tolerate. Once you get that to acceptable levels, the temperature rise is usually low enough.
       Also consider that 2 oz copper and more is available. The extra cost of specifying 2 oz copper for outer layers may be less than making the board larger. or dealing with the heat or voltage drop. 2 oz on outer layers doesn't usually add that much cost. If you stitch together a trace on both outer layers. you have 4x the copper cross section than for a single trace of 1 oz thickness. If it's only one or two traces in a otherwise low current design. you can leave the soldermask off the trace and have a copper wire soldered over the trace. There are actually bus bars meant for this. But, consider the manufacturing cost. 2 oz copper may start to look like the cheap option when you consider the total cost of alternatives.
      Again, look at all the options and all the criteria for deciding on trace width. Don't focus on temperature rise. or assume that thicker copper is more expensive once the whole system considered.
      The number you need called "MOT" (maximum operating temperature). for the laminate you have in mind. You also need to know what the internal temperature will be. in your product (including the heat the relay connections add). For FR-4 to maintain electrical properties. that might be 130°C (a bit higher for mechanical properties). If the maximum temperature in the vicinity of your PCB will not exceed 60°C you could allow for 70°C rise. For FR-2 it might be 105°C, so the limit might be 35°C rise.
      More normal practice is to allow for 20-30°C to keep the PCB from getting all discolored and weak over time. and there might be issues with component life if you add too much heat. It's conceivable UL approvals might complicated with internal temperatures exceeding 105°C.
      There are some very nice higher temperature laminates with Tg = 170°C available. and higher MOT, but it's cheaper to use 2oz copper.
      Other options are to pull back the solder mask. and parallel the conductors with solder (assuming wave soldering). or to solder a jumper wire in place of, or in parallel to, the conductor. If you can keep the conductors short in length, most of the heat will get sunk out of the pins. Look at the design of any PC power supply for ideas, about every penny has pinched in their design.
      I'd say 50-75C would be MAX acceptable amount for me. (But I still would not be happy). Do you have other options? if you are creating heat you have resistance and are wasting too much power doing nothing useful.

What is Tg value in PCB?

      Printed circuit boards, even standard FR-4 PCBs, are resilient pieces of electronics. There are certain conditions for which these boards will not be appropriate. For example, PCBs for aerospace may subjected to extreme temperatures. both very high and very low temperatures. For situations requiring PCBs that can handle low-temperatures, also called cryogenic PCBs. special low-temperature PCB material may be necessary.
      The typical FR-4 PCB should be able to withstand temperatures close to -50°C. At this point. you may start to find brittle cracks in the material. While this is quite cold, you will generally. prefer your FR-4 PCBs not to get anywhere near this standard to cut stress. and lengthen the life of your PCB. Furthermore, in industries like the aerospace industry. where PCBs may end up in outer space. the board may exposed to temperatures as low as negative 150°C. far. below the recommended temperature level for FR-4 PCBs.
      you can use many materials as a substitute to FR-4. when making PCBs that need to be resistant to extreme temperatures. Polyimide materials, for example. are resistant to temperature on both ends. standing up to temperatures not only cryogenic. but as hot as 260 degrees Celsius, as well as being fire-resistant. Polyimide is susceptible to water, but, which is an important design consideration. A more expensive option is ceramic, where you have the best of both worlds. Ceramic PCB’s are no susceptible to water and can withstand extreme high and low temperature. The down side to cermaic PCB’s are their cost and design barriers.
      Another useful material in cryogenic PCBs is aluminum. which can conduct at .5 Kelvin, or negative 272°C. The downside of aluminum is that it is reactive. and so it requires unconventional methods when building circuit boards.

Heat can be damaging

      Printed circuit board (PCB) materials  formulated to withstand a certain amount of heat. but when the temperatures rise beyond certain limits, circuit performance can suffer. especially at higher frequencies. Heat-tolerant PCB materials. and  considered circuit designs can tolerate a certain amount of heat. if a circuit designer is aware of the various parameters. that best describe a circuit material’s behavior when temperatures rise.
       Heat can come from various sources and affect circuits in different ways. especially as circuit boards  assembled with increasing density in efforts to make smaller. lighter circuit designs. Heat can  generated by a component mounted to the circuit board. or from a source external to the circuit board. Designers of high-power radar systems are. familiar with the large amounts of heat generated by vacuum-tube amplifying devices. such as klystrons and traveling-wave tubes (TWTs).

       More recently, high-density amplifying semiconductors such as gallium nitride (GaN) transistors. mounted to a PCB can produce hotspots. besides to raising the power levels of RF/microwave signals. Heat sources external to a PCB, such as in automotive electronic systems. can also raise circuit temperatures. and pose reliability issues. Designing circuits that are  affected. by such heat sources. is a matter of understanding the behavior of RF/microwave circuit materials. at higher temperatures.
       Heat causes most materials to expand, including circuit materials. Because of the smaller wavelengths at higher-frequencies. microwave and especially millimeter-wave (30 GHz and higher) circuits have small features. that can become distorted as a circuit board expands with higher temperature. Besides, due to growing demands for smaller electronic designs. many circuits  designed with circuit materials having higher dielectric constants. that yield smaller circuit features for a given frequency and wavelength. High temperatures cause expansion of circuit materials. which can change the form of transmission lines. and alter the impedance of conductors from a desired value,  50 Ω. The undesired results for circuits at higher temperatures include loss of linearity, distortion. even shifts in frequency due to changes in transmission-line dimensions.  


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