Stainless Steel World Magazine December 2023 | Page 28

Figure 3 . Tensile strip test results

Metal Strip

Molten Solder

Solder

Stainless Steel

Downward

Stainless Steel

Force on A

E

Metal Strip Time

D

B C

Upward Figure 4 . Example of how solderability tester works .

was used to pull on the parts until there was a break in the solder connection , or a break in the base metal strip .

Solderability tester The solderability tester measures the interfacial force between molten solder and the specimen being soldered ; in essence , measuring the ability of the solder to flow on a surface . A common definition of solderability is “ the ability of a metal to be wetted by molten solder ”. The solderability test is unique in that it permits the simultaneous evaluation of base metal , solder , flux , and temperature to determine the effect of these factors in the soldering process . Three of these factors are held constant , while the fourth is the measured variable . In the test , a fluxed metal strip specimen is held above the molten solder and connected to a load cell . The solder pot is raised to immerse the specimen to a preset depth below the solder for a set period of time . The load cell measures the weight change as solder draws up the test specimen . The weight difference is converted to wetting force by an on-line computer analysis . The solder pot then is lowered at the end of the immersion cycle , pulling the specimen out of the solder . To evaluate the results , the height of the curve within the immersion time of the solderability test dip is measured in terms

A

1 88.0 lbs ./ per sq . inch 2 97.5 3 86.0 4 88.0 5 95.0 6 105.0 * 7 84.0 8 85.0 9 94.0 10 96.0

B / C D E / F

F

of wetting force . The wetting force is a measure of the extent of solder flow . Greater solder flow will result in better soldering .

Solderability test terms Time to cross axis ( TCA ) - Time , in seconds , needed to achieve positive solder wetting . This is the measure of how fast the solder will start to spread on a surface . Solder wetting ( in µ N / mm ) at given Times - Wetting force expressed in micronewtons / millimeter (µ N / mm ) at the time indicated in the solderability curve . Maximum wetting ( MW ) - Maximum force ; expressed in micronewtons / millimeter (µ N / mm ), in the positive solder wetting region of the solderability curve . This is a measure of how far the solder will spread on a surface . Integrated area under the curve ( IA ) - The area between the curve and the x-axis during the entire immersion cycle . The area is recorded in units of micronewtons / millimeter-seconds (µ N / mm-sec ). This measure combines the effects of both maximum wetting and time to cross axis to give a single performance value .

Solderability test results : Comparison of standard liquid stainless steel flux to best stainless steel soldering pastes

Solder Wetting ( in µ N / mm ) at given Times

Force on Metal Strip

Buoyancy

Downward

Upward

Maximum Wetting ( E )

Figure 5 . Description of how to read solderability testing results .

28 Stainless Steel World December 2023 www . stainless-steel-world . net

Time

G G

H

A

B

H

C

Steel broke before the solder joint

E

Integrated Area Under the Curve

F

G