As the efforts continue towards meeting RoHS requirements and
the elimination of lead by mid 2006 for some countries, finding the
ideal surface finish is on most manufacturers’ minds. The surface
finish has to be lead free and more important should be able to
produce a reliable solder joint when assembled at high temperature
with a lead free solder.
Presently there are a series of alternate surface finishes in use
throughout the printed circuit industry. Some finishes are widely
used and others are used for very specific applications.
Gold based Finishes:
- Electroless Nickel /Immersion Gold ENIG
- Electroless Nickel/Electroless Palladium/Immersion Gold ENEPIG
- Direct Immersion Gold (DIG)
- Organic Solderabilty Preservatives OSP
- Immersion Silver IAg
- Immersion Tin ISn
- Selective OSP/ENIG, DIG/ENIG
Most of these finishes have a well documented record of solder
joint reliability over time as they have been used for many years
with eutectic Sn/Pb solder. The key question now is the solder
joint reliability of these finishes if they are assembled at higher
temperatures using lead free alloys.
This article will present lead free (SAC 305 alloy) solder joint
reliability data with the gold finishes namely ENIG, ENEPIG and
DIG. SAC 305 is an alloy composed of Sn with 3.0%Ag and
0.5%Cu. SAC alloy has a melting point temperature of 218°C
compared to Sn/Pb at 183°C.
Solder balls of SAC alloy and Eutectic Sn/Pb were soldered to
BGA pads finished with the different surfaces under investigation.
The soldered coupons were then placed in an oven at 150oC from
0 to up to 1000 hours to simulate aging. At different time intervals
samples were removed and subjected to ball pull testing.
In addition SEM cross sections were also examined.
Sample Preparation for ENIG, ENEPIG
and DIG deposition
Sample coupons for evaluation were prepared as follows:
copper-clad laminate (FR4), was acid copper plated to 1.0 mil
thickness. BGA pads of 0.6 mm diameter were defined with
soldermask., using commercially available products. A set of
coupons were then plated with each of the following surface
finishes. Ni-7%P/Au (ENIG) (5μm/0.05μm), (ENEPIG)
(5μm/0.06μm/0.03μm) and DIG (0.04μm).
Evaluation Method of Solder Joint and Results
On the various ENIG and ENEPIG sample substrates, solder balls
of Sn/37Pb and Sn/3.0Ag/0.5Cu were mounted, followed by heat
treatment at 150oC in air for 0, 100, 300, 500, and 1000 hours
to conduct ball pull tests. For the DIG samples only the SAC alloy
balls were mounted and ageing was simulated up to 500 hours
for ball shear testing and 1000 hours for cross section analysis.
The conditions of ball pull tests are shown in Table 1.
Table 1. Ball pull test conditions
Solder ball 0.76mm, Sn-37Pb Solder (Senjyu)
0.76mm, Sn-3.0Ag-0.5Cu Solder (Senjyu)
Pad Diameter 0.6mm
Flux 529D-1 RMA type (Senjyu)
Reflow equipment TMR-15-22LH (Tamura)
Ball Pull Dage 4000
Ball Pull Speed 170μ/sec
*Reflow temperature: peak temperature of 240°C
Figure 1 Testing Method schematic
Figure 2. Results of ball pull test when Sn/37Pb was used
Figure 3. Results of ball pull test when Sn/3.0Ag/0.5Cu
Figure 1 shows a schematic of the experimental sequence.
and the results of ball pull tests are shown in Figure 2 (in the case
of Sn/Pb solder) and Figure 3 (in the case of Sn/Ag/Cu solder).
As shown in Figure 2, in the case of Sn/Pb solder, the change of
failure mode which indicates a change of the joint interface showed
no clear difference when ENIG film was used, but occurred
remarkably when ENEPIG was used.
As shown in Figure 3, when Sn/Ag/Cu solder was used, in contrast,
minimum change of failure mode was observed in the case of ENIG
film, but no change of failure mode was observed in the case of
ENEPIG film, exhibiting excellent characteristics.
SEM examination of the different IMCs (intermetallic layers) show
that the IMC thickness in general grows over time at elevated
temperature. This growth does not impair the solder joint strength
of ENIG, ENEPIG or DIG under the conditions of SAC alloy soldering.
As shown in Figure 5, eutectic Sn/Pb SEM examination explained
why the ENEPIG failed prematurely with aging. There was irregular
areas of excessive IMC growth. This is believed to be the result of
the incompatibility of Pd and Pb (no IMC formation). In contrast the
IMC formed with ENEPIG and SAC alloy had superior strength
and limited thickness. It is theorized that the Pd layer
minimized/controlled the diffusion of the Ni into the Sn.
Figure 6 shows the intermetallics formed after ageing
a solder joint with DIG.
Figure 4. DIG Ball Pull vs Time
For DIG Figure 4, the failure mode did not exhibit significant loss of strength over 500 hundred hours of age simulation.
Figure 5. SEM photographs (1000X)of IMC
Figure 6. EPMA analysis of DIG with SAC alloy soldering
1000 hrs at 150°C
Both ENIG and ENEPIG form Ni/Sn intermetallic solder joints with
the SAC alloy. DIG on the other hand forms a Cu/Sn intermetallic.
ENIG performed very well with both eutectic Sn/Pb and
with SAC alloy.
ENEPIG in this investigation proved not to be a suitable finish for DIG performed real well with both eutectic (not included in this
eutectic Sn/Pb soldering as was previously believed; it does however
produce a superior Ni/Sn solderjoint with SAC alloy
study) as well as with lead free SAC alloy. The absence of Ni in
the DIG finish renders it unique for applications where Rf signals
are propagated and where Ni may cause an interference. Unlike
other immersion processes, DIG solder joints are not prone
You may also be interested in this: Study of Suitable
Palladium and Gold Thickness in ENEPIG Deposits for
Lead-free Soldering and Gold Wire Bonding
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