IPC / SMTA Cleaning Workshop November 16, 2010

Transcription

1 Electrical Failures IPC / SMTA Cleaning Workshop November 16, 2010

2 Content Technology Innovation Device Interactions Tin Whiskers Soil Effects Complexities

3 Rapid Technology Innovation More performance in smaller platforms High Speed Processors Lead-Free Soldering Reliability critical driver

4 Reliability The capacity of a device to perform as designed For specific interval under stated conditions Minimal failures The Goal: Performs as promised every time

5 Device Interactions

6 Device Interactions Mostly IPC standards Mostly SEMI standards Surface Mount Technology -Solder pastes -Fluxes -Coatings Advanced Packaging -Photoresists -Plating chemicals Semiconductor Fabrication -Wafer Bumping -Flip Chip -Package on Package JEDEC Standards Mackie, A. (2009, Oct). Electromigration Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

7 Increase in Hardware Failures Gaseous contamination Chemical effects Creep Corrosion Mechanical effects Compressive Stress Tin Whiskers Optical signal interference Friction Electrical effects Circuit impedance Dendritic growth Arcing ASHRAE (2008). Gaseous and Particulate Contamination Guidelines for Data Centers.

8 Reliability Manufacturers are in a constant struggle to Maintain the reliability of their hardware Every Shrinking feature sizes Decreased distance signals travel White residue flux migration

9 Miniaturization Heat dissipation Air flow exposes circuits to gaseous contamination Non-hermetic sealed packages Moisture entrapment Decreased spacing Voltage differences increase the risk of ion migration Compressive stresses Corrosion Smaller component features reduces the distance corrosion needs to travel ASHRAE (2008). Gaseous and Particulate Contamination Guidelines for Data Centers.

10 Co-Planar Board Finishes Coplanar board finishes are especially susceptible Immersion silver plating (ImAg) over copper Organically Coated Copper (OSP) Silver and copper are highly reactive to sulfur Levels as low as 3 ppm is enough to induce corrosion Silver is more noble metal than copper In the presence of atmospheric water, forms an electrochemical cell

11 Sulfur Bearing Gases Even in absence of moisture can Attack copper and silver Migrate as silver and copper sulfide Breeches exposes underlying board finishes Corrosion grows and leads to electrical opens

12 Cathode Silver e - e - Galvanic Cell Galvanic cell potential from copper and silver metal reduction Copper, being more active metal, represents the anode when electrochemical reactions with silver take place Copper corrodes faster than other based oxidizing environment. Electron / Metal Atom Flow e - e - e - e - e - e - e - M - + Mackie, A. (2009, Oct). Electromigration Our Mutual Friend. SMTA IWLPC, Santa Clara, CA. e - M e - e - Anode Copper

13 Tin Whiskers

14 Tin Whiskers Lead-Free = High tin alloys Whiskers bridge the gap between conductors Cause shorts Carry enough current to stop the circuit from functioning correctly Copper Trace Lead-free Solder + Whisker _ Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in SAC 305. SMTA IWLPC, Toronto, CA.

15 Whisker Growth Whiskers grow to relieve compressive stress in tin Compressive Stress Whiskers grow by atomic diffusion in a crystalline manner Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in SAC 305. SMTA IWLPC, Toronto, CA.

16 Whisker Location Greatest Incidence of whiskers on the edges of the traces This is probably because Greatest concentration of flux residue Exposed copper / thin tin coating Howell et al. (2010) Effect of Soldering Method and Flux Type on Tin Whisker Growth in SAC 305. SMTA IWLPC, Toronto, CA.

17 Soil Effects

18 Assembly Contamination Stencil cleaning Wet solder paste Chip bonder adhesives Bare Board Oxides Ionic contaminants Flux residues Organic acids Rosin Resin structures Polymers Functional additives

19 Flux Residues A Common Enemy Stencil Printing Soldering FLUX Underfill Conformal Coating Mackie, A. (2009, Oct). Electromigration Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

20 Soil Effects Electromigration: Electrochemical Migration (ECM) Surface Mount Technology: Movement of ions under a potential gradient Closely associated with SIR (Surface Insulation Resistance) BETWEEN adjacent metal conductors Electromigration (EM) Semiconductor / Packaging: Movement of atoms caused by electrons flowing through a metal WITHIN a metal conductor Mackie, A. (2009, Oct). Electromigration Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

21 Electrochemical Migration No-clean, Halogen-free Fluxes Mackie, A. (2009, Oct). Electromigration Our Mutual Friend. SMTA IWLPC, Santa Clara, CA.

22 Tin Salt Migration Partially removed flux residue under component Dendritic growth Creep corrosion

23 Flux Residue Migration under Conformal Coating

24 Flux Left under Components Feature size reduction Increase failure risks

25 Ion Migration in an Electric Field Propagated by The charge balance at the interface Current density entering and leaving the device Electrolyte from moisture causes ions to split and form dendrites Cathode Anode

26 High Voltage Source: FET1_Drain_Joint_Failure_ _3_jpg.htm

27 Shorting / Sparking / Failure

28 Electrochemical Migration Key Factors Contamination Soils Gaseous Moisture (%RH) Adjacent metallic conductors Electric field (potential gradient) Ionizable metals Hydrophilic / mildly-hydrophilic continuous phase

29 Failure Region

30 Conclusions Reliability of hardware is more prone to risk due to ever shrinking feature sizes Move to Lead-Free increases risk factors Co-Planar Board Surfaces Creep Corrosion Tin Whiskers Flux residues bridge conductions Electromigration risks Removal and mitigation of contamination sources is increasingly important to assure reliable devices that perform as promised

31 Questions

32 Mike Bixenman Kyzen Corporation Desk Cell Author