Application testing of SOLDERPASTE – Surface Finish interactions can be quantified using a special HALT Test vehicle developed by TLS
Background The July 2006 Regulatory deadline for Lead-Free assembly is rapidly approaching.  The industry is seeking consensus on a standard Lead-Free Alloy for surface mount reflow process.  SAC Alloy is being adopted by many CEM/OEM(s).  However, the best PCB surface finish is still being debated.  Everyone agrees that the lowest-cost finish option is OSP; however, post reflow test problems may require that we deploy a probe-compatible alternative.   Immersion TIN is a leading candidate in Europe.   Asian PCB suppliers favor immersion SILVER but they are also capable of providing both (Sn & Ag) finishes on demand.   Even now, there are those who propose electrolytic-flash GOLD as a solution.     The goal of our test program is to cut through this haze and provide your engineering management with pertinent data on which to base their decision when choosing among the many paste and surface finish options available in the industry.
THE SOLDER JOINT – During reflow, the fluid solder alloy, influenced by temperature profile and solderpaste flux activity, wet-out the surface finish on PCB and components, dissolving the finish into the melt.  Ultimately, a slightly altered alloy solidifies to form the solder joint.  These key variables directly impact the fillet geometry and interfacial metallurgy of surface mount solder connection.
HALT TEST The TLS-HALT test vehicle and protocol has demonstrated its ability to sense the interaction between solderpaste and PCB/Component surface finish.   Our test subjects pairs of identically formed solder-joints to 9 different thermo-mechanical strain conditions and senses the accumulated damage needed to induce failure as an electrical open.
The photo shows a typical HALT test platform loaded into the Temperature/Vibration test chamber.  The assembly is mounted in an inverted position onto the fixture.  The test sequence begins and continues until failure eventually occurs and all components fall away from the assembly.
Electrical continuity is continually monitored and recorded for each component using external scanning equipment connected via ribbon cable.
A typical OSP Copper component is attached to the FR4 substrate at two locations with a span between joints of 25, 50 or 75 mm.  Differential expansion, caused by CTE mismatch between Cu-FR4, results in a predictable Thermal cycle strain. Different Component heights of 10, 18, or 25 (mm) produce predictable, G’s induced, bending strain within the solder joint during vibration.  These two factors  - thermal cycle and vibration-induced bending -  populate the 3x3 strain matrix with 3-fold replication.  Each platform is populated with 3x3x3 = 27 components.
Shape of Typical OSP Copper Component
Example Component Placement on PCB Test Platform
TEST CYCLE -The HALT test cycle combines thermal cycling and step vibration.  The Graphic below shows the test protocol.   48 Rapid Temp cycles between -250C and 850C are applied during the first 200 min while the G-level is increased in 5 GRMS steps.     Some alloys and pastes do not completely fail in this first stage of the test; therefore, if required, STEP STRESS is increased above 25 G's up to a maximum of 50 GRMS