RF/Microwave Die Attach of Gallium
Nitride Devices Achieving Less Than 1%
Voiding in a Flux-Free Environment
Bruce Wilson
SST International Downey, CA
IMAPS
Chesapeake Chapter
Winter Technical Symposium
February 19, 2014
Conditons
• Gold Tin preform material No Flux Used
• Small table top reflow vacuum solder
station
• Customer provided samples and
undersized preforms
• Process profile techniques to reduce
voiding
• SAM results provided by customer
Gold Tin Material
• 80Au 20Sn flat sheet form 2 mils thick
• Melting temperature 278°C
• After initial reflow subsequent melting the alloy
becomes more gold rich and the melting point is
increased drastically
• Review phase diagram
• Disadvantage and Advantage
AuSn Melting Point Changes
•
•
•
•
2% deviation at 18% tin melting point 350°C
4% deviation at 16% tin melting point 400°C
Reference : Indium Corp. of America
Disadvantage : Cannot reflow at same
temperature
• Advantage : AuSn can be used for both die
attach and lid seal on same package
AuSn Properties
• Does not scavenge gold as conventional tin-rich
solders
• Tin content is tied up in gold-tin intermetallics
• Forming Au5Sn during the soldering operation
• Gold wetting reaction proceeds at a slow rate
and scavenges gold less than Sn rich solders.
• Thus more gold is available to prevent
underlying nickel diffusing to the gold surface
Unique Aspects of AuSn Solder
• Visual appearance: shiny bright smooth silver
color – indicative of proper reflow temperature
• Dull rough or gold colored appearance –
indicative of over-heating excessive reflow
temperature
• AuSn preforms do not require any chemical acid
treatment to remove surface oxides as required
by other Indium, Lead, or Tin based solder
alloys
• AuSn is brittle and not easily trimmed or cut
Key Requirements for Gold Plating of
Substrates and Lids
• No Brighteners - additives added to the gold plating bath
to enhance color appearance
• Highest quality gold is needed
• SST plating requirements:
• Underlying nickel 50 to 150 micro inch nickel sulfamate
per QC-N-290A followed by
• Gold 1.25 microns thick Type III Grade A (99.9%
minimum) per ASTM B488-01 Maximum Knoop
hardness of 90
• Low phosphorous content 4 to 7% good for soldering
wetting but high level 12 to 14% does not allow solder
wetting and can cause micro cracking in the solder
interface
Equipment
Issue with AuSn Sized Preforms
• All AuSn supplied preforms 0.002 inch thickness
• Supplied preforms not sized for either GaN or bond
support die
• 2 hand trimmed undersized preforms used side by side
for each GaN die
• Possible voiding from space between hand trimmed
preforms during reflow
• Typically preform is a single piece at 90-95% of die area
for 1 to 2 mil thick preform material of AuSn
• Substrate too thick for X Ray : required machine shop to
remove about 50 mils of substrate for X Ray for 1st part
• X Ray indicates essentially void free solder interface
Preforms & Die (GaN & bond support)
GaN device 102 mils by 37 mils
Bond support die 130 mils by 30 mils
Hand trimming of supplied preforms was required for assembly
Used Hand Trimmed Multiple Preforms
Non Ideal Case
Typical
preform
size
design is
90 to
95% of
die area
Using
multiple
preforms
not
advised
Die Located Inside Graphite Insert
With Bezel for GaN die
Programmed Profile Used
Run time 16 mins
Void Reduction Features Used in the Reflow Profile
After Run 1 310C 45 Seconds Reflow
Solder has good shiny appearance
Milled Backside of Thick Substrate To be
Able to Use X Ray
X Ray of Milled Substrate Run 1
Standard X Ray
Contour X Ray
Photos of Several Packages
Run 4 Part 4A
Run 5 Part 5A
Run 6 Part 6A
Run 7 Part 7A
CSAM Scans of Voids
Conclusion: SAM Void Summary of 20 Die
(10 GaN Die & 10 Bond Support)
Even with non ideal preform size. SST used two
undersized preforms for each GaN die. SST still
achieved an Average Total Void of < 0.6 %
THANK YOU
for your attention