Selecting the right solder alloy for your application is crucial to the success of the project. If the right solder alloy is not selected and plating requirements are not defined, the selected process may not be capable of producing solderable deposits. Incomplete plating requirements often result in excessive rework and scrap.
With so many different alloy compositions of solder available, choosing the right alloy could be quite challenging.
The best approach is to consider:
1) What is the maximum temperature that the device(s) can be exposed to without damaging the device or device performance?
2) Consider that some alloys really need from 25°C to in some cases 100°C hotter (above the melting point) to flow well and wet the mating surfaces. Can the device tolerate this elevated solder temperature?
3) What are the die and substrate metallizations?
4) Do you have the correct metallizations for that selected solder alloy?
5) Is there a good history of successful good wetting and flow for that alloy?
6) Flux Less or with Flux?
7) Is there a lead free requirement?
For flux-less die attach and lid seal the workhorse solder alloy used by the majority of SST Vacuum Reflow Systems' customers is 80Au20Sn with a melting point of 278°C. This alloy can be used as received without the need for any surface oxide removal as compared to lead, tin, or indium based solder alloys where the native metal oxides would interfere with solder wetting and would require acid etching just prior to loading for soldering.
Below is a list of three Lead Tin Solder and Other Alloys:
Below is a list of Lead Free Solder Alloys:
Since there are numerous plated material/plating requirement combinations, only the more commonly used combinations are addressed in this blog. The following callouts will provide solderable plated deposits for the solder alloys shown:
1) For solderable gold plate when gold-tin, tin-lead, lead-indium, tin-indium, lead-tin-silver, lead-indium-silver or tin-silver solder alloys are used:
Underplate with solderable nickel sulfamate per QQ-N-290, 50-150 microinches thick.
Followed by:
Gold plate per Mil-G-45204,Type III, Grade A, 50-100 microinches thick (Class 1).
2) For solderable gold plate when gold-germanium solder is used:
Wood’s nickel strike per QQ-N-290, 5-50 microinches thick. The nickel plate thickness shall not exceed 50 microinches.
Followed by:
Gold plate per Mil-G-45204,Type III, Grade A, Class 2, 150-250 microinches thick.
3) For solderable silver plate when lead-tin, lead-indium, tin-silver, tin-indium, lead-tin-silver or lead-silver solder alloys are used:
Underplate with solderable nickel sulfamate (no brighteners) per QQ-N-290, 50-150 microinches thick.
Followed by:
Plate solderable silver per QQ-S-365, Type I or II, Grade B, 300-500 microinches thick.
4) For electrolytic nickel plate applications when lead-tin, lead-indium, tin-silver, tin-indium, lead-tin-silver or lead-silver solder alloys are used:
Plate solderable nickel sulfamate (no brighteners) per QQ-N-290, 150-300 microinches thick.
5) For electroless nickel plate applications when lead-tin, lead-indium, tin-silver, tin-indium, lead-tin-silver or lead-silver solder alloys are used*:
Plate solderable nickel-boron per AMS-2433,Type 2, 150-300 microinches thick.
*Electroless nickel-phosphorus plate shall not be used for soldering applications. If the phosphorus content should exceed 10%, the resulting deposit may be nonsolderable.
Refer to chart on page 15 in “Principles of Soldering and Brazing” by Giles Humpston and David Jacobson showing the solder spreading characteristics for solder alloys based on Tin, Indium, Lead, Bismuth, and Silver. This chart shows that certain alloys require much hotter reflow temperatures above the melting point to achieve good solder flow characteristics.
Fluxless or with Flux
If your goal is low voiding, then fluxless soldering is preferred. Solder paste has solvents and flux additives that outgas and create more bubbles/voids as compared to solid preforms.
Download these resources for more information:
| Plating Requirements technical paper | SST Model 3130 Data Sheet |
SST Model 1200 Data Sheet |
 |
 |
 |
----
Zap (Pierino) Zappella
Process Development Engineer
SST Vacuum Reflow Systems