Elimination of H2S in immersion tin plating solution

A process for depositing tin or tin alloys onto copper features on a printed wiring board substrate through immersion plating, said process comprising:

• contacting the copper features with an immersion tin plating solution comprising:
• i. divalent tin ions, wherein the divalent tin ions comprise tin sulfate or tin methane sulfonate, wherein the concentration of divalent tin ions is between 5 g/L and 20 g/L;
• ii. an acid, wherein the acid is used to maintain a pH between about 0 and about 5 in the plating solution;
• and
• iii. a complexing agent, wherein the complexing agent comprises thiourea, wherein the concentration of thiourea is between about 60 g/L and about 120 g/L;
• wherein the printed wiring board substrate is immersed in the immersion tin plating solution to deposit tin on the copper features at an initial plating rate of at least about 0.08 μm/minute to deposit a thickness of about 0.5 to about 1.5 μm of tin on the copper features;
• wherein hydrogen sulfide builds up in the plating solution during use, causing plating quality to decrease and a plating rate to decrease more than ten percent from an initial plating rate, wherein nitrogen gas is bubbled through the plating solution to remove hydrogen sulfide from the plating solution,
• whereby the plating rate and plating quality can be restored or maintained, and
• wherein the plating solution is capable of maintaining a plating rate within ten percent of the initial plating rate of the plating solution after at least 1000 operating hours.