[ Solder ]
[ Solder ]
Solder paste for stainless steel
A new zinc chloride-based solder paste that works on hard-to-solder 316 stainless steel alloy has been developed . The paste consists of powdered solder alloy suspended in flux paste with binders . The solder paste can be heated rapidly by torch yet is stable enough to be heated for long periods of time in an oven . The bond strength of the soldered connection is very strong and , in some cases , exceeds the strength of the underlying steel .
By William F Avery , Dr . Y . Baskin , and Rebecca Zronek , Superior Flux & Mfg . Co ., USA
( a ) ( b ) ( c ) ( d )
Figure 1 . Solder Test ( a ) Paste 1 ( b ) Paste 2 ( c ) Paste 3 ( d ) Paste 4
A stable stainless steel soldering paste has been needed for some time . While we have great liquid and paste fluxes for soldering stainless steel , we haven ’ t had stable and sufficiently strong solder paste products to do this work . There are several considerations regarding this :
••
Simple conversion of flux in paste form to solder paste does not work because the chemistry of these fluxes will often result in the flux attacking the solder powder owing to the activity of the flux .
••
An ideal stainless steel soldering paste can be heated rapidly by torch .
••
An ideal stainless steel soldering paste can be heated in long heating processes such as in ovens and be able to solder connections after long heating periods .
••
The stainless steel used in this test was 316 stainless .
••
The solder used for evaluation was 96.5 / 3.5 tin-silver , often used for stainless steel connections .
••
The completion of this test compares the resultant new proposed solder paste to the best liquid flux we have for stainless steel soldering , and to another alternate zinc chloride-free stainless steel soldering paste . chloride ) to maximize the mixture as a paste . One mixture was found to be stable with the solder powder . Deciding on an Activator Package - Step 2 : None of the formulations , when made into solder paste , were capable of soldering stainless steel , suggesting that other chemical activation schemes were needed . Deciding on an Activator Package - Step 3 : The next test looked at altering the activating scheme and changing the paste consistency . A different acid choice was used along with other activators . The paste chemistry was altered to make a more fluid flux paste formulation . Figure 1 shows how the pastes looked when soldered to stainless steel using hot plate heating at 350 ° C . The results were quite dramatic ; see figure 1 ( a ) - ( d ).
Reducing solder paste splattering Because of the chemistry used with this solder paste , it was necessary for some water to be present . Pastes made in the previous section had intentionally too much water added , simply to ease the creation process . The series of successful fluxes created in this round were then tested via the solderability tester to measure their viability . The best visual ( the least amount of splattering ) and mixing result ( went into solution the easiest ) was found to be the new solder paste shown in Figure 2 .
Testing the new soldering paste The new soldering paste has been tested as described previously , by a simple tensile test , and by solderability testing . The test results are shown in Figure 3 .
Tensile strength A simple tensile strength test was carried out to measure the overall strength of the solder bonds with the new stainless steel soldering paste . An interference joint was created by soldering one strip of stainless to another using the solder paste and torch heating . The tensile tester
New development work Deciding on an Activator Package - Step 1 : Development was concentrated on improving soldering to stainless steel . Several formulations were developed to determine the best mixture of ingredients ( particularly activators beyond the zinc
( a ) ( b )
Figure 2 . ( a ) Liquid flux and solder alloy ( b ) New solder paste
www . stainless-steel-world . net Stainless Steel World December 2023 27