Ratios in Metal Plating

3 common, “rule of thumb” ratios I come across regularly in metal plating deal with bright acid tin, spray washing, and high metal alkaline zinc.  This article will briefly discuss each “rule” and offer suggestions based on real life scenarios to determine if they are “rules”, helpful suggestions, or flat out myths.  It is advised that you contact your supplier before making any changes to your existing metal plating system, but these may be helpful when analyzing your baths and considering the efficiency, distribution, and/or bath life of the chemistry in your process.

Bright Acid Tin Plating Solutions:  Common control parameters for a bright acid tin plating system range from a 3:1 (acid:tin) to a 5:1 (acid:tin) ratios.  Our typical acid tin baths in the field and new make-up solutions are blended and maintained at a 5:1 ratio of acid to tin (10% sulfuric acid:2 OPG tin metal),  and I have heard a 5:1 ratio is where tin baths should be maintained.   Tin baths are very forgiving baths to operate and control and I have seen this ratio maintained over a wide range in several instances, so it is not entirely a “rule of thumb” for plating acid tin.

When a bath appears to be dull or lacking in other quality issues, it seems to be most operators’ desire to reach for the proprietary organics and chuck in brightener, wetter, or some other wide range of additives labeled with letters (A,B, C) or catchy phrases like “booster”, “adjuster”, “corrector”.  Prior to adding any proprietary additives, perform a quick analysis and make a simple addition of sulfuric acid or tin salts to reach the 3:1 through 5:1 ratio.  It is my experience this commonly fixes many of the problems the bath may be experiencing.  Another word of caution, just because the recommended vendor concentrations may advise operating the bath at 10% sulfuric acid and 2 OPG tin metal (5:1 ratio), and your bath is at 0.8 OPG tin metal, do not assume you need to immediately raise the concentration of tin metal to 2 and sulfuric acid to 10%, or dilute the concentration of acid to 4% (5:1 ratio “rule”).  In many cases we have simply adjusted the metal to be above 1 OPG and the quality issue was addressed.  Then later, slight adjustments with proprietary chemicals were made (determined by hull cell analysis) to better improve bath expectations and performance.

Bottom line bright acid tin plating is an extremely forgiving and solution with a wide operating range of tin metal and acid concentrations.  I would suggest using the ratio as a common set point and helpful guide to maintain your existing bath, but not as a mandatory “rule”.

Alkaline Spray Wash Cleaners:  Another common “rule of thumb” I have seen recommended by vendors, and in some instances stated on their Technical Data Bulletins for a “rule” to control their spray cleaner solutions – “…when the cleaning solution reaches a 2.5:1 ratio of total:free alkalinity  it is no longer effective and needs to be dumped…” (I have also seen 2:1 as well).  The total:free alkalinity ratio refers to the amount of total alkalinity – alkalinity provided by all chemistry to the amount of free alkalinity – the amount alkalinity supplied by free OH molecules (commonly supplied by NaOH and/or KOH).

I believe this is a completely useless ratio to control and maintain spray cleaner solutions.  I have commonly run spray cleaners at a variety of total:free alkalinity ratios without signs of poor cleaning, or less efficient cleaning from a new cleaning solution make-up.  The primary contaminate used to consider when a cleaner is ready to be dumped is the soil load, followed by many other factors (ability to perform a readable titration color change, maintenance schedules, sludge build up in tanks, nozzle blockage, etc.), not the total:free alkalinity ratio.  Total alkalinity will climb in any spray washer without any introduction of soil; just by introducing air by agitation to the solution (running it through spray nozzles) will increase the amount of sodium carbonates (a source of total alkalinity).  The presence of sodium carbonates will not lessen the cleaning effectiveness of our spray cleaners.

High metal Alkaline Zinc Plating solutions:  Maintain the ratio of caustic to zinc metal at a 10:1 ratio if you are running a high metal alkaline zinc plating solution.  I commonly use this ratio to best balance a high metal, high efficiency alkaline zinc plating bath.  When the customer calls with issues of plating distribution, poor LCD coverage, and/or deposit qualities, I first ask what is the ratio of caustic to Zn metal.  Typically it is either out of balance, or in some cases low on proprietary additives.  However, before determining if an addition of an additive is required by lengthy hull cell testing, if the bath is significantly low in caustic or Zn metal, I always balanced to a 10:1 (caustic:Zn) ratio first, prior to hull cell analysis or many other observations and testing.  A majority of the time this slight and simple adjustment will fix the problem of distribution, poor LCD coverage, and/or deposit qualities.  It is true some proprietary packages do enhance and improve distribution once the ratio is obtained, but the first step to running towards higher efficiency and distribution is to get the ratio in balance.