lead free solder on regular HASL pads

do not 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

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If you are looking for more details, kindly visit lead-free hasl.

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.

Definitelymix 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

With the continuous development of the electronics industry, the technical level of PCB is also rising. Common surface treatment technology include HASL(Lead Free HASL and HASL), immersion gold, gold plating, OSP, etc. HASL is to coat a layer of tin-lead alloy on the copper surface to prevent the copper surface from being oxidized, and then provide a good soldering base for subsequent assembly. Do you know the difference between lead free HASL and HASL?

1. Lead-free HASL belongs to environmentally protection process and does not contain harmful substances "lead"; HASL is not an environmentally protection process and contains harmful substance "lead", which is harmful to human body for long-term use.

2. The lead content of lead free HASL shall not exceed 0.5, but the lead content of HASL shall reach 37.

3. From the surface of the tin, HASL is brighter and lead-free HASL is darker; The wettability of HASL is better than lead free HASL.

4. Lead will increase the activity of the tin solder wire during the soldering process. Compared with the lead free solder wire, the lead-containing solder wires is easier to use. Lead-free HASL has a higher melting point than HASL, and the solder joints will be much stronger.

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5. The melting point of lead free HASL is about 218 degrees, the temperature of the tin stove needs to be controlled at 280-300 degrees, the temperature of wave soldering needs to be controlled at about 260 degrees, and the temperature of reflow soldering is about 260-270 degrees.

6. The melting point of lead free HASL is about 183 degrees, the temperature of the tin stove needs to be controlled at 245-260 degrees, the temperature of wave soldering needs to be controlled at about 250 degrees, and the temperature of reflow soldering is about 245-255 degrees.

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