DirectFET® MOSFET: Underfill and Coats
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Conformal Coating and Device Underfill
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Conformal Coat
(to improve resistance to moisture)
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Underfill
(to improve t/c performance)
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"Play Audio
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Underfill and Conformal Coats
- Extensive testing has been carried out on generic materials that may be encountered for both applications
- Underfills: in most cases these will turn DirectFET into something that it almost indestructible
- Conformal coatings: do not in any way match the performance of underfills. There are many differing types of conformal coatings and all react differently.
For further details refer to the Application Note AN-1050
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Conformal Coating and Device Underfill T/C Results, Post 1000 Cycles
RDS(on) Shift Post 1k T/C [mOhm]
Note: Conformal coating is not recommended
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Temp Cycle Performance
| Substrate |
Pb or Pb-free |
Temp Cycle Range |
Mean RDS(on) Shift |
Normalized RDS(on) Shift Over 1000+ Cycles |
| FR4 |
Pb |
-40°C/+125°C |
130uOhms |
1.0 |
| FR4 |
Pb-free |
-40°C/+125°C |
180uOhms |
1.3 |
| FR4 |
Pb-free |
-55°C/+150°C |
380uOhms |
2.8 |
| AlSiC or Cu IMS |
Pb |
-40°C/+125°C |
260uOhms |
1.9 |
| AlSiC or Cu IMS |
Pb-free |
-40°C/+125°C |
190uOhms |
1.4 |
| AlSiC or Cu IMS |
Pb |
-55°C/+150°C |
270uOhms |
2.0 |
| AlSiC or Cu IMS |
Pb-free |
-55°C/+150°C |
310uOhms |
2.3 |
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| FR4 with C. Coat |
Pb |
-40°C/+125°C |
300uOhms |
2.3 |
| FR4 with Underfill |
Pb |
-40°C/+125°C |
3uOhms |
0.02 |
Standard criteria is less than 20% degradation of thermal impedance. For DirectFET MOSFETs, a more sensitive test is a 20% shift of Rds. 20% of 3.4 milliohm = 680 micro-ohms.
As the above table shows, the worst case is ˝ of that level.
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Conformal Coating and Device Underfill t/c Results
- THB - all devices passed 1000hrs
- HTRB small die in ‘M’ can - showed ‘opens’ – Air trapped between the can and the board expanded during test and pushed the DirectFET can away from the board causing 'open' connections. Care should be taken when conformally coating the MQ outline that large volumes of air are not trapped
- HTRB Large die in ‘M’ can OK
- T/C - standard devices shifted an average of 120mOhms
- T/C - underfilled devices were the best as expected and shifted an average of only 50mOhms over 1000cycles
- T/C - conformally coated devices were the worst and shifted an average 350mOhms due to the trapped air ‘popcorning’ effect when air trapped between the can and the board expands during test and pushes the DirectFET can away from the board.
- As outlined in AN-1050, International Rectifier cautions against using conformal coatings on small die devices without the use of underfill.
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Lead-free and DirectFET MOSFETs
- Extensive testing has been carried out on Pb-free solder for thermal cycling properties
- In almost all cases Pb-free solder will result in improved resistance to electrical and thermal degradation during temperature cycling
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IR’s proprietary DirectFET® technology is covered by US Patent 6,624,522 and
other US and foreign pending patent applications.
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Details of test card
