AN533 STMICROELECTRONICS | Alldatasheet
Document overview
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Technical content
Datasheet sections
- 1 Through-hole packages
- 1.1 Thermal resistance
- 1.2 Thermal impedance
- 1.3 Insulating materials
- 1.4 Insulated components
- 1.5 Handling and mounting techniques
- 1.6 Through-hole package wave soldering
- 2 Surface mount packages
- 2.1 Thermal resistance
- 2.2 Thermal impedance
- 2.3 Mounting techniques and R th(j-a)
- 2.4 Reflow soldering information
- 3 Revision history
SCRs, TRIACs, and AC switches, thermal management precautions for handling and mounting Introduction The behavior of a semiconductor device depends on the temperature of its silicon chip. This is why electrical parameters are given at a specified temperature. To sustain the performance of a component and to avoid failure, the temperature has to be limited by managing the heat transfer between the chip and the ambient atmosphere. The aim of this note is to show how to calculate a suitable heatsink for a semiconductor device and the precautions needed for handling, mounting and soldering techniques.
AN533 Through-hole packages
1 Through-hole packages
1.1 Thermal resistance
1.1.1 Review
The thermal resistance of semiconductor assembly is the parameter which characterizes its resistance to the heatflow generated by the junction during operation. A temperature exceeding the maximum junction temperature curtails the electrical performance and may damage the device. The maximum dissipated power capability is: Where:
- Tjmax is the maximum junction temperature of the semiconductor in degrees ( °C)
- Ta is the ambient air temperature in degrees ( °C)
- Rth(j-a) is the thermal resistance between junction and ambient air in °C/W The Rth(j-a) takes into account all materials between the junction and ambient air. An analogy between Ohm’s law and the thermal equivalent circuit can be made:
- Electrical resistance corresponds to thermal resistance
- Current corresponds to dissipated power
- Voltage corresponds to temperature Thus: V = R . I corresponds to ΔT = Rth . P
1.1.2 Dissipated power for a thyristor or a TRIAC
The maximum power dissipation versus average on-state current (for SCRs) or RMS on- state current (for TRIACs) is given in the datasheet for each product. However, a more accurate result is obtained by using the Vto and Rd values with the following calculation: P = Vto . IT(AV) + Rd . I2 T(RMS) Where:
- Vto is the threshold voltage specified in the datasheet
- Rd is the dynamic on-state resistance specified as Rd in the datasheet
- IT(AV) is the average on-state current
- IT(RMS) is the RMS on-state current Figure 1 shows the RMS and average values for different waveforms of current. Pmax = Tjmax -Ta Rth(j-a)
Figure 1. RMS and average currents
1.1.3 Dissipated power in a TRIAC
sum of the dissipated power of both thyristors. For a phase angle conduction the RMS current is given in Figure 1 C.
1.1.4 TRIAC without external heatsink
Figure 2 shows the thermal equivalent diagram for a TRIAC without external heatsink.
- Ta: ambient air temperature where the TRIAC is located
- Rth(j-a): thermal resistance between junction and ambient air given in the datasheet
- P: dissipated power in the TRIAC depending on the used TRIAC and on the load current The following equation defines the junction temperature depending on these parameters: Tj = P . Rth(j-a) + Ta IpA B C Ip Ip T/2 T/2 T T Tt0 t t t IT(AV) = 2 . Ip .t 0 .Tπ IT(AV) = IT(AV) = 1 - + . sin 2 . Ip π 2 . Ip . I T(RMS) =² Ip² T IT(AV) = i(t)dt T I T(RMS) =² i²(t)dt T T I T(RMS) =² Ip².t 0 2 . T I T(RMS) =² Ip 2 . π 2 . t0 T 4 . . t 0 T π ( ( (( ( (cos t 0 .² π T π P = 2 . 2√ π .I T(RMS) .V t0 + Rd .I T(RMS)
Figure 2. Thermal equivalent diagram
1.1.5 TRIAC with external heatsink
specified in the datasheet, a heatsink has to be used. defined. Figure 3 shows the thermal equivalent diagram. Figure 3. Thermal equivalent diagram with external heatsink
Through-hole packages AN533 The formula to calculate the thermal resistance between heatsink and ambient air is the following: Where:
- Tj is the junction temperature in °C
- P is the maximum dissipated power in W
- Rth(j-c) is the thermal resistance between junction and case in °C/W Rth(c-h) is the thermal resistance between case and heatsink in °C/W, depending on the contact case/heatsink. Since the current alternates in a TRIAC, we have to consider the Rth(j-c) in alternating current which is different to the Rth(j-c) in direct current. This difference is due to the die of the TRIAC. The first half of the silicon die works when the current is positive, the second when the current is negative. Because of the thermal coupling between these two parts, this gives the following equation. R th(j-c)AC = 0.75 . Rth(j-c)DC
1.1.6 Choice of heatsink
Choosing a heatsink depends on several parameters; the thermal characteristic, the shape and the cost. However, in some applications a flat heatsink can be sufficient. Figure 4 shows the curve Rth(h-a) versus the length of a flat square heatsink for different materials and thickness. Some applications need heatsinks with an optimized shape where the thermal resistances are not known. For this, the best solution involves measuring the case temperature of the component in the worst case scenario and keeping to the following formula: Tc < Tjmax - P . Rth(j-c) Where:
- Tc is the case temperature
- Tjmax is the maximum junction temperature
- P is the dissipated power in the component
- Rth(j-c) is the thermal resistance between junction and case. Rth(h-a) = Tj -T a P - Rth(j-c) - Rth(c-h)
Figure 4. R th(h-a) versus the length of a flat square heatsink
1.1.7 Forced cooling
be required. Heatsink manufacturers give a coefficient depending on the air or liquid flow.
1.2 Thermal impedance
In steady state, a thermal equivalent circuit can be made only with thermal resistances. when the component is on during a time lower than the time to reach the thermal resistance. th/Rth plotted against pulse duration.
Figure 5. Thermal transient impedance of a BTA08-600SW.
1.3 Insulating materials
on both sides. Because of its rigidity it can be easily broken. need contact grease. However, they can be easily broken, as they are less fragile than mica. combination of the mica + grease.
1.4 Insulated components
- ceramic between the die pad and the heatsink of the component (TO-220AB / TOP3 / RD91 packages)
- resin used for encapsulation (ISOWATT220AB / TO-220FPAB packages) All insulated packages delivered by STMicroelectronics are in accordance with UL1557 recognition applicable for "electrically isolated semiconductors". The added material increases the thermal resistance between the junction and the case, but the total thermal resistance (R th(j-a)) is lower than the one when using a non insulated component with an external insulating material. In addition, it simplifies assembly and reduces the cost. For two 16 A TRIACs in TO-220AB package, Rth(j-c)AC values ( °C/W) are shown in Table 2:
1.5 Handling and mounting techniques
affect the long term reliability of the device, or even damage it.
1.5.1 Bending and cutting leads
on the die. This damage can affect the long term reliability of the devices. Table 1. R Table 2. Comparison of R th(j-c) for sample insulated and non-insulated products
- Never bend the leads laterally ( Figure 9).
Figure 9. Bending the leads laterally (not allowed)
- Never bend the leads more than 90° and never bend more than once.
- Make sure that the bending / cutting tool does not damage the leads.
1.5.2 Using a heatsink
- The mounting surface should be flat, clean and free of burrs and scratches.
- The use of a thin layer of thermal silicone grease ensures a very low contact thermal resistance between the component and the heatsink. An excessively thick layer or a excessively viscous silicone grease may have the opposite effect and cause the deformation of the tab.
- The planarity of the contact surface between device and heatsink must be very low (less than 50 µm for TO-220AB). Insertion If the heatsink is mounted on the PC board, it should be attached to the component before the soldering process of the leads. Mounting techniques Mounting must be done carefully. Excessive stress may induce distortion of the tab and as a consequence mechanical damage on the die. Soldering: It is not recommended for through-hole packages. Incorrect
Figure 14. Relative variation of R th(c-h) compared with Rth(c-h) at 0.6 Nm versus Table 3. Recommended torque and thermal contact resistance
- For BTB20-xxx, BTB24-xxx and TYNxx40, the maximum torque is 0.5 Nm.
AN533 Through-hole packages
1.6 Through-hole package wave soldering
The lead-free through-hole devices may be soldered with lead-free solder pastes or alloys (Sn-Ag-Cu based alloys). The typical soldering temperature is 260 °C. Alternatively these devices may be soldered with SnPb based solder pastes. The soldering temperature is then typically around 220 °C. Interface adherences on through-hole package structures are qualified to sustain only 3 consecutive dips of their connections in a solder pot at 260 °C (-0 °C / +5 °C) .
- immersion duration: 10 seconds each
- delay between 2 dippings: 5 minutes
- minimum distance solder to package plastic body: according to packages, by default 1m m Lead-free devices are described in an internal specification defining:
- their characteristics: lead-free connection coating, solderability and identification
features
- their reliability such as soldering resistance, reliability performances, whiskers risk prevention. This specification is available for ST customers upon request (Title: ECOPACK Components Definition & Characteristics). Please consult that document for further information. ECOPACK is a registered trademark of STMicroelectronics
2 Surface mount packages
2.1 Thermal resistance
ambient temperature, mounting conditions etc...). Figure 15. Rth(j-a) versus copper surface under tabs (FR4 board -
2.2 Thermal impedance
- tp < 10 ms: die influence
- tp < 0.1 s: package influence
- tp < 10 s: PCB influence
- above 10 s: thermal exchange board-air (example: with / without forced cooling) Figure 16 shows the Zth / Rth ratio for SMD packages and Figure 17 shows Zth(j-a) for DPAK and D2PA K. 0123450 100 110 120 130 S(Cu) (cm²) Rth(j-a) (°C/W) SOT-223 DPAK D2PAK 02468 1 0 1 2 1 4 1 6 1 8 2 00 100 S(Cu) (cm²) Rth(j-a) (°C/W) 0 4 8 1 21 62 02 42 83 23 64 00 S(Cu) (cm²) Rth(j-a) (°C/W)
Figure 16. Relative variation of thermal impedance junction to ambient versus pulse Figure 17. Typical Z th(j-a) for DPAK and D2PAK
2.3 Mounting techniques and R th(j-a)
- FR4 - Copper
- IMS (Insulated Metal Substrate)
- FR4 board with copper filled through holes + HEATSINK
- IMS + HEATSINK. 1E-3 1E-2 1E-1 1E+0 1E+1 1E+20.01 0.10 1.00 tp(s) Zth(j-a)/ Rth(j-a) 1E-3 1E-2 1E-1 1E+0 1E+1 1E+2 5E+20.01 0.10 1.00 tp(s) Zth(j-a) / Rth(j-a) SOT-23 SOT-223 0.001 0.01 0.1 1 10 100 1,0000.1 0.2 0.5 Zth(j-a) INFINITE HEATSINK IMS FLOATING IN AIR FR4 die package board board/air influence of the various mounting elements
Figure 18. Mounting techniques for power SMDs
- use of large heat spreader areas (heat sink) at the copper layer of the PCB
- use of copper-filled through holes in addition to an external heatsink for an even better thermal management However, due to its power dissipation limitation, using the FR4 board with these techniques is advisable only for currents up to 8 A max. A technology available today is Insulated Metallic Substrate (IMS). This offers greatly enhanced thermal characteristics for surface mount components. IMS is a substrate consisting of three different layers:
- base material which is available as an aluminum or a copper plate
- thermal conductive dielectric layer
- copper foil, which can be etched as a circuit layer. Even if a higher power is to be dissipated, an external heatsink can be applied leading to an Rth(j-a) of 4.5 °C/W (see Table 4). This is commonly applied in practice, leading to reasonable heatsink dimensions. Often, power devices are defined by considering the maximum junction temperature of the device. In practice, however, this is far from being fully exploited. The designer should carefully consider which is appropriate mounting method (see Table 4) to be used according to the dissipated power. The type of board will influence the thermal performance of the system. Table 4 shows the R th(j-a) depending on the mounting techniques for DPAK and D²PAK. Copper foil Insulation Aluminium / copperFR4 board Copper foil Footprints Copper foil FR4 board Heatsink Copper-filled through-holes Copper foil Insulation Aluminium / copper Heatsink
2.4 Reflow soldering information
The surface mount assembly is a 4-step process.
- component placement on the board
The soldering process causes considerable thermal stress to a semiconductor component. 2PAK package the maximum temperature is 245 °C. standard for further information about “small” and “large” component definitions. necessary temperatures to achieve good soldering. contact the whole package slug during the process. Table 4. R
Surface mount packages AN533 Figure 19. ST ECOPACK recommended soldering reflow profile for small packages For each individual board, the appropriate heat profile has to be adjusted experimentally.
- Always preheat the device. The purpose of this step is to minimize the rate of temperature rise to less than 3 °C per second (recommended 2 °C/s) to minimize the thermal shock on the component.
- Dryout section, after preheating, ensures that the solder paste is fully dried before starting reflow step. Also, this step allows the temperature gradient on the board to be evened out.
- Peak temperature should be at least 30 °C higher than the melting point of the solder alloy chosen to ensure the reflow quality. In any case the peak temperature should not exceed 260 °C. Lead-free devices are described in an internal specification defining:
- their characteristics: lead-free connection coating, solderability and identification
- their reliability such as soldering resistance, reliability performances, whiskers risk prevention. This specification is available for ST customers upon request (Title: ECOPACK Components Definition & Characteristics). Please consult that document for further information about reflow and wave soldering. Voids pose a difficult reliability problem for large surface mount devices. Such voids under the package result in poor thermal contact and the high thermal resistance leads to component failures. Coplanarity between the substrate and the package can be easily verified. The quality of the solder joints is very important for two reasons:
- Poor quality solder joints directly result in poor reliability.
- Solder thickness affects the thermal resistance significantly. Thus, tight control of this parameter results in thermally efficient and reliable solder joints. 01234567 Time (min) Temperature (°C) 2°C/s recommended 6°C/s max 220°C 125 °C 260°C max 255°C 180°C 90 sec max 10-30 sec 90 to 150 sec 3°C/s max 01234567 Time (min) Temperature (°C) 2°C/s recommended 6°C/s max 220°C 125 °C 260°C max 255°C 180°C 90 sec max 10-30 sec 90 to 150 sec 3°C/s max
3 Revision history
Table 5. Revision history