Ask Helena & Leo

Question: I have a customer who seems to be having issues making acceptable solder connections using the vapor phase process. Can you tell me if there are any specific issues related to vapor phase soldering to immersion tin plated pads on relatively thick multilayer PCBs?

Answer: This is a very interesting question regarding the reflow of immersion tin plating.

I’ve found and attached this web site that discusses this particular issue.

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4684411&url=http%3A%2F%2Fieeexplore.ieee.org%2Fstamp%2Fstamp.jsp%3Ftp%3D%26arnumber%3D4684411

Although the temperature of the vapor phase material is high enough, the problem appears to be the tin oxide on the surface of the board which is preventing the reflow of the solder. To address this issue the flux has to be more active and have the ability to not break down at vapor phase reflow temperatures.

I would suggest getting in touch with the solder paste manufacturer and evaluating a more robust flux material in the solder paste rheology composition.

I’ve copied the abstract for your information, complements and copyright of IEEE:

“…The focus of this work is the tin oxide characteristics and their influence on solderability. A good wetting result can be only achieved if the oxides are completely removed. Due to this fact different flux systems are used to remove the oxide layers and other contaminations from the surface. In order to supply sufficient flux for complete removal of tin oxides, knowledge about the oxidation state of tin oxides on new and on aged immersion tin final finishes were obtained. The oxidation process of the immersion tin surface on PCBs was investigated under different reflow cycles with lead-free temperature profiles and different soldering atmospheres. The characterization of the thin oxide layers were studied by using high resolution analysis methods as XPS, TEM, and SERA. The results proved a thickness of immersion tin oxide layers at approximately 7 nm by TEM and XPS analysis after plating. Furthermore, an amorphous mixture of SnO and SnO2 were initially found. Additionally, the examined immersion tin surface on SMD 1206 resistor pads did not show a remelting after the first reflow cycle although the pads experienced a peak-temperature of 250degC and the melting point of tin amounts to 231.8degC. A further solder test presented a considerable decrease in the wetting behavior after reflow aging. In this work is shown that this behavior results from thicker oxides, which were formed by the influence of high temperatures and oxygen atmosphere. Furthermore, a TEM examination indicated slight semi-crystalline tin oxide regions after two reflow cycles under nitrogen atmosphere.”

As can be seen the peek temperature experienced was 250C, which is above the melting point of tin at 231.8C. This is where the proper flux must be chosen to handle the temperatures during this process.