Vapor phase vs. convection reflow in RoHS-compliant assembly
2000 1000 hrs.
Visual inspection for solder balls,
Cycles Accel. Age
tombstones, bridging, voids and dewetting
SnPb No Clean Convection
indicated no apparent difference between
19.5 * 9.34 *
the two methods of solder joint creation.
Vapor Phase
6.67 16.8 No tombstones were experienced on the
Lead-free No Clean Convection
2.11 9.16
JOCY test vehicle boards in either case.
Visual inspection indicates that while
Vapor Phase
2.59 5.5
vapor phase created solder joint perfor-
SnPb Water Soluble Convection
14.5 No data
mance and micro-section appearance on
the board is very good, it might be a good
Vapor Phase
10.2 7.96
idea to explore increasing the lead-free
Lead-free Water Soluble Convection
1.31 23.7
TAL above the 60 to 90 seconds recom-
Vapor Phase
mended by solder paste manufacturers
4.16 5.16
to accommodate thorough heat transfer
* Tin-Lead No-Clean resistance measurements tended to decrease slightly. Others in-
to larger components or clusters of large
creased or were mixed hence, absolute values to were used to assess changes.
components. Larger thermal load compo-
nents, especially in clusters, tend to retard
Table 1. Average percent resistance change (absolute value).
the complete melting of lead-free paste.
It is more difficult to ensure good joints
on components with high thermal mass
Initial 2000 Cycles 1000 hrs.
in convection processing because while
Thermal Shock Accel. Age
trying to achieve a sufficient TAL on larger
SnPb No Clean Convection
19.1 23.1 23.6
components, smaller components in less
populous areas may tend to overheat.
Vapor Phase
22.8 21.6 19.0
Much discussion in trade magazines
Lead-free No Clean Convection
27.6 25.6 26.6
and forums such as the IPC TechNet has
Vapor Phase
focused on the question of soldering tin-
27.3 27.6 26.2
copper and SAC-alloy-terminated BGAs
SnPb Water Soluble Convection
23.7 20.3 and other components with standard tin-
Vapor Phase
lead solders. Using a 230˚C vapor phase
23.5 20.5
system, even liquification of these termina-
Lead-free Water Soluble Convection
26.3 27.6 25.0 tions ceases to be a problem while posing
Vapor Phase
26.9 28.0 25.7
little chance of overheating heat sensitive
components. Similarly, risks associated
Table 2. Shear/tensile force required to remove SOIC16 (pounds of force).
with lower T substrates and temperature
g
sensitive components is reduced relative to
lead-free convection processing.
Test Description profiles (Figure 2) provided a peak tem-
Since cleanliness had been studied
Tests were conducted using a VP reflow perature of 245˚C and a TAL in the 60-90
using ion chromatography for a previously
process. Vehicle boards were used with tin- second range recommended by paste sup-
published report
2
, a cleanliness compari-
lead HASL or lead free immersion silver, pliers. Vapor phase soldered boards were
son was made for this report using ROSE
immersion tin or ENIG surface finishes, as soldered in an EPM-IBL SLC500 vapor
techniques. An Omegameter operating
appropriate. Boards were populated with phase soldering chamber using Galden
above 100˚F was employed. No differences
tin-lead or lead-free components, printed, LS/230 Perfluorinated heat transfer
were detected in ionic cleanliness between
assembled and soldered using standard fluid. The vapor phase profiles developed
boards soldered using convection reflow
reflow or VP production equipment. The provided a TAL of about 90 seconds and
and those soldered in vapor phase. Lead-
solder pastes selected for testing included a maximum temperature of 230˚C, a
free no-clean samples tended to have 50%
tin-lead and lead-free no-clean and water temperature that is governed by the vapor
higher contamination levels than standard
soluble formulations. Assembled test temperature. After a vapor phase profile
tin-lead boards due to the type and level
boards were thermal shocked between is established, TAL can be modified to
of flux used in lead-free pastes. All results
-45˚C and +125˚C with 20 minutes dura- achieve any time required without exceed-
were well below IPC limits.
tion at each limit for 500, 1000 and 2000 ing the 230˚C maximum temperature.
Resistance across soldered BGA daisy
cycles in EPIC’s Failure Analysis Labora- The vapor phase equipment first
chain arrays of 40 and 80 joints were the
tory. Other test boards were subjected to preheats the board using infrared. Next,
same for convection and vapor phase
accelerated aging at 85˚C and 85% relative the work is lowered into the vapors at a
reflowed test boards within the limits of
humidity for 1,000 hrs. The JOCY test programmed rate to regulate ΔT and TAL.
experimental measurement. (Daisy chained
vehicle is populated on only one side, al- After the work reaches the maximum va-
dummy 169 and 352 termination BGAs
though it is equipped with plated through- por temperature, the duration of its expo-
containing four daisy chains each were
holes (PTH) for mixed technology tests. sure is preprogrammed. Several soldering
used.) Solder joint conductivity did not ap-
Test boards were populated with dum- programs can be developed by the engineer
pear to deteriorate measurably after either
my 402, 603, 805, 68 pin PLCC, TSOP32, and stored in memory to suit the needs of
2000 thermal shock cycles or 1000 hours
SOIC TQFP QFP208 and daisy-chained different lead-free or tin-lead board types.
of accelerated aging at 85˚C/85RH. Dur-
BGA169 and BGA352 components. ΔT and TAL are controlled by the program
ing the 2000 thermal shock cycles and
Standard lead-free convection reflow developed by the engineer.
Continued on page 34
22 – Global SMT & Packaging – March 2009
www.globalsmt.net
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