Chemical Silver(Immersion Silver = Imm Ag)Lead-Free PCB´s-The Surface of the Future!

Lead soldering is no longer permitted, and the use of alternative surface refinements for pcb´s has so farprovided unsatisfactory results, meaning that the lead-free problem has yet to be resolved. The chemical nickel-gold procedure (Imm NiAu) was unsuccessful due to high costs, as was chemical tin. Both procedures have proven to be very complicated and sensitive in the further course of processing management. Lead-free solderings also have serious disadvantages, as hot air tin surfacing exposes the multi-layers to higher processing temperatures and copper leaching effects. We would briefly like to explain why we believe chemical silver is the lead-free procedure of the future, and why it will be the focus of our future investments in the field of surface refinement:

UNPROBLEMATIC WASTE WATER PROCESSING

Similarly to chemical tin surfacing, it is a non-electrical, auto-catalytic procedure based on the contrasting electronegativityof copper and the more “precious” silver. The advantagesof this processing manner compared with chemicaltin are fewer processing steps and lower rinsing temperatures(30 – 50 C°).

From the printed circuit board manufacturer’s viewpoint, onestrong point in favour of the silver procedure is the fact thatwaste water processing is unproblematic, whereas Imm Aghas had problems with toxic gold chemistry and chemical tindue to the use of thiourea.

BUT WHAT ADVANTAGES ARE THERE FOR YOU AS

THE USER?

Not only is the chemical silver refinement uncomplicatedand reliable, but once the manufacturing parameters havebeen set they can be dependably utilised for furtherprocessing and continued use.

Due to the thin surface layer – a thickness of 0.15 – 0.3 mµis recommended –the surface of the pad is smoother andtherefore there are, even in fine pitch areas, norestrictions when assembling. The silver functions merelyas a protection against oxidation, and the actual solder pointis between the copper andchemical tin, therefore the solderpoint is absolutely stable. It fulfills all UL requirements(UL796 specif.) and meets the usual requirements of automobile manufacturers:

Test results

Test                     AlphaSTAR             Result   Requirement

SIR 85°C/85%RH                       2.46+9     24h                          Passed     >1E+8;IPC-4553                    

IPC-TM-6502.6.3.5                     3.15E+9   96h                          Passed

SIR 35°C/85%RH 100VDC      3.56E+11  24h                         Passed      >1E+10; IPC-4553 and GR 78 (Bellcore)

IPC-TM-650 2.6.3.5                    4.62E+11  96h                         Passed

EM 10V bias, 100V Test            2.46E+9   24h initial                                   Resistance no higher than the Power of 10

85°C/85%RH                              1.18E+10  504h (21days)      Passed     Dendrites not Identified

IPC-TM-650 2.6.14.1                  Dendrites not Identified         Passed     IPC-4553 and J-STD-004

lanic (static method)                  Typically 0.07-0.48 µg/cm²    Passed     <1.56 µg/cm² Equivalent NaCL

IPC-TM-650 2.3.25.1                  of Equivalent NaCl                                     IPC-4553 and J-STD-001 Part 8.3.6

The soldering effectiveness is excellent, allowing multi-fittings with wave soldering and re-flow procedures becausesilver is aneffective diffusion barrier to the copper below.The procedure is alsosuitable for pressing techniques. Due to the “soft”processing methods, there is no need for anysubsequent solder stop degradation, thereby avoiding unpleasant surprises whenfitting.

Manufacturers give a 12 month (minimum) storage guarantee but the conditions for perfect storage should be noted (<35 C° and< 85% humidity). Under these conditionsa longer storage period is alsopossible. In case of any subsequent solderingdifficulties, it is possible to repeat the silver processing.