The test vehicle designed for this qualification is shown in Fig. 1. The Printed Circuit Board (PCB)
was built with high Tg-FR4 epoxy glass material. The thickness was 0.093” with four signal layers
and two ground layers and contains 6-mil blind microvias in the construction. The surface finish
used has been immersion Ag, OSP and ENIG. The board dimensions are 7.5” X 10”. All locations
on the PCB have daisy chain connections for monitoring electrical continuity. All BGA and CSP
locations were designed with multiple test points to assist in isolating any solder joint opens to a
particular row. The site would also receive the proper lead-free SMT reflow profile to use as seen in
Fig. 2.
As a part of the internal development process, both ATC and MDS testing were performed on
the corporate built lead-free test vehicle assemblies in order to benchmark the MDS against
the widely accepted ATC testing. Both MDS and ATC show similar trends in the reliability
results (Fig. 3). For any Solectron site qualification, only MDS testing was performed in order
to save time and cost.
Note: 0 to 100C, 40-minute Accelerated Thermal Cycling condition was used. Tested as per
IPC-9701.
Fig. 4 shows the failure data for 0.5mm pitch CSP plotted on a lognormal scale. Corporate
data and failure data from other SLR sites are also shown in the same plot. From the plot, it
can be seen the first-pass lead-free assemblies from the Solectron site performed significantly
better than the corporate tin-lead cells and slightly better or equivalent to the corporate leadfree
test cells. We are looking for each site to perform along the same curve. We validate the
results with cross-section analysis and solder joint evaluation study.
After evaluating the assemblies as per the acceptance criteria, the materials characterization lab
will then provide a report and issue a certificate to the site that met the criteria requirements based
on benchmarking of corporate assembled boards or provide a detailed description of areas of
concern with corrective actions.
Cross-sections are taken of the various devices assembled by the sites to insure good wetting and
proper solder joint intermetallic formation. A few cross-section examples are shown in Fig. 5 and 6.
The pull and shear force values recorded on the Solectron corporate test vehicles are shown for
reference in Fig. 8 (both with Sn/Pb and SnAgCu solder) versus results on lead-free solder
obtained from boards built at Solectron Shenzhen, China. The pull forces recorded at the Solectron
site test vehicles were in the same range, but the shear forces were a little lower than the baseline
corporate values. These results were used only for relative assessment with no hard criteria set,
considering the nature of the testing (i.e., operator dependency, limited sample size and different
ageing conditions).
Lead-Free Wave Solder Process Site Qualification Procedure
The guidelines and criteria in this document were established from the results of Solectron DFSS
Lead-Free Wave Project, iNEMI lead-free project, and other internal lead free studies. All
manufacturing sites are required to participate in this LF wave process verification activity to obtain
corporate endorsement for lead-free wave assembly. Solectron developed processes for our
existing equipment that needed upgrades to be able to accept lead-free Sn3Ag0.5Cu alloy or
processed with any new equipment purchased. Solectron has implemented mainly nitride coating
stainless steel, called “Melonite”, but future upgrades will use titanium and equivalent parts to
replace the original stainless steel parts. By having a set test vehicle for the sites to build, they
have been able to prepare early in their readiness for their customer requirements no matter which
solution is needed.
Procedure Overview
In order to start the lead-free wave assembly process site verification, the qualifying site or division
needs to have a lead-free wave verification kit. The material set required for the testing includes
twelve 0.093” thick mm. Ag SLR through-hole
test vehicles, plus another five 93mil thick
boards with OSP surface finish. The OSP
board surface finish added more holefill
challenges for the lead-free wave assembly,
and it helps the site to gain more experience
on lead-free wave assembly. The corporate
team supplies wave solder processes and
indicates the thermocoupled lead-free wave
profile as a guide to run the assemblies as
seen in Fig. 8.
All assembled test boards will go through 100% visual and X-ray inspection. Some components are
selected for cross sectioning and pull testing. After evaluating the assemblies as per the
acceptance criteria which are based on the corporate test vehicle results, the materials
characterization lab will then provide a report and issue a certificate to the site that met the criteria
requirements based on benchmarking of corporate assembled boards or provide a detailed
description of areas of concern with corrective actions.
The main concern for lead-free wave soldering is the potential increase in soldering defects.
Among already known defects such as reduced hole fill, increased bridging, voiding or solder
balling, the exact formation mechanisms and the effect of potential solder joint concerns such as
shrinkage holes, fillet lifting and pad lifting are still not well understood.
The PCB is 5.5” X 7”. It is designed to be manufactured in 0.062”, 0.093”, 0.125” and 0.180” for
development purposes. For qualifying the sites we used 0.093 to help them set-up their processes.
It has 16 layers and varying lead-to-hole area ratios with different number of copper planes
attached to different pin locations. The sites wave solder lead-free DIPs, axial resistors, headers
and connectors to gather data to compare to the corporate developed lead-free wave soldered
data. |
