lead free solder on regular HASL pads

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Caution must be used when using tin / bismuth alloys. It is dangerous to mix tin / bismuth with lead containing alloys. Tin, bismuth, and lead can form a very low melting combination that melts around 95 °C. This could potentially lead to solder joint failure due to natural heating of the assembly during use.
 
Tin / bismuth alloys are safe to use in combination with other lead free tin-based alloys. In some cases surface mount assemblies are made with SAC305 on the first side and tin / bismuth on the second side. The low reflow temperature for tin / bismuth minimizes intermetallic growth in the SAC305 solder joints and allows for soldering of thermally sensitive components.

Definitely mix Pb solder with the low temp Bismuth-based solders (also there is the different melting point issues, but the alloy that forms is awful). Joint becomes very brittle due to strong intermetallic growth and easy to break with even what I could consider moderate force. For example, with the right (or wrong) mix of solder and a decent grip, I can pull SMD aluminium polymer caps off the board with my bare hands. I'm pretty sure remember reading a super old IBM article where they investigated this Bi/Pb mix for some temperature sensitive parts in a mainframe, and it was a disaster. Can't find the article now thoughAlso from http://www.surfacemountprocess.com/low-temperature-lead-free-solder-paste.html

The process requirements in the PCB production are very important, which directly determines the quality and positioning of a board. Such as HASL, gold plating pcb, ENIG.

 Lead-Free HASL is similar to standard HASL, but with an obvious difference… It doesn't use Tin-Lead solder. Instead, Tin-Copper, Tin-Nickel or Tin-Copper-Nickel Germanium may be used. This makes Lead-Free HASL an economical and RoHS compliant choice.

Advantages of Lead-Free HASL

Due to the good quality of ENIG, the price is higher too. Many customers choose the most common HASL process.

• On the surface, Lead-tin is brighter and lead-free tin (SAC) is dim.
• Lead in HASL is harmful to humans, but lead-free not. The lead eutectic temperature is lower than lead-free, which depend on Lead-free alloy composition. For example, The eutectic of SNAGCU is 217 degrees, the pcb soldering temperature is eutectic temperature plus 30 to 50 degrees. The lead eutectic is 183 degrees. The mechanical strength and brightness with lead is better than lead-free.
• Lead-free HASL is an environmentally friendly process. It is very harmful to the human body. It is also a process advocated at this stage. The lead content in lead-free tin is not more than 0.5, and the lead-free HASL has a high melting point, so that the solder joint is firm. In essence, lead spray tin and lead-free tin spray are a process. Only the purity of lead is different. Lead-free tin is a more environmentally friendly and safer environment for the human body, and it is also a development trend in the future. It is recommended that you use it. The characteristics and shortcomings of the two types of process hasl and lead-free hasl are introduced. Among them, hasl is easy to use, but it is unsafe, environmentally friendly and harmful to the human body. Therefore, it is recommended to use lead-free hasl, non-toxic and harmless, is also the surface treatment process currently advocated.

Caution must be used when using tin / bismuth alloys. It is dangerous to mix tin / bismuth with lead containing alloys. Tin, bismuth, and lead can form a very low melting combination that melts around 95 °C. This could potentially lead to solder joint failure due to natural heating of the assembly during use.
 
Tin / bismuth alloys are safe to use in combination with other lead free tin-based alloys. In some cases surface mount assemblies are made with SAC305 on the first side and tin / bismuth on the second side. The low reflow temperature for tin / bismuth minimizes intermetallic growth in the SAC305 solder joints and allows for soldering of thermally sensitive components.

Definitely mix Pb solder with the low temp Bismuth-based solders (also there is the different melting point issues, but the alloy that forms is awful). Joint becomes very brittle due to strong intermetallic growth and easy to break with even what I could consider moderate force. For example, with the right (or wrong) mix of solder and a decent grip, I can pull SMD aluminium polymer caps off the board with my bare hands. I'm pretty sure remember reading a super old IBM article where they investigated this Bi/Pb mix for some temperature sensitive parts in a mainframe, and it was a disaster. Can't find the article now though. Also from http://www.surfacemountprocess.com/low-temperature-lead-free-solder-paste.html