Poor Wetting of Terminations and Pads

Non-wetting or insufficient wetting is also encountered. It must be understood that different metallization will exhibit differing spread and wicking characteristics and also flux activity will play an important role. Lead-free SAC alloys during solderability testing using wetting balance instruments demonstrated the best wetting when water washable flux systems were used. No-clean flux systems containing less activator and/or free of halides demonstrated lower wetting speeds and lower maximum force readings.

Bare copper OSP boards, which have seen more than one thermal cycle, are prone to incomplete pad wetting. While pure tin, silver immersion finishes exhibit better solder spread. Ni/Au if the nickel is not affected with impurities or oxides will normally solder well. Below are two examples, one with SAC alloy on copper and the other on silver immersion; both QFP’s were reflowed in air, using a SAC no-clean paste ROLO type flux.

SAC and bare copper OSP	SAC and Silver Immersion

Poor solderability, insufficient wetting, poor wicking of solder, and large contact angles can also result from an inadequate thermal profile. It is very important to achieve good thermal equilibrium across the whole board, this becomes more important with lead-free since the peak temperature window is narrower. SAC alloys melts at 217 °C while the peak temperature needs to be in the range of 235-245 °C.

If BGA’s are present on the lead-free assembly, these components act as heat sinks, the solder paste may not completely reflow under the BGA, while other smaller components.

May show good soldering. It becomes very important to establish good thermal profiling points across the board, including under BGA’s. To properly insure wetting has occurred completely, optical inspection or X-ray inspection may be necessary.

	A test board is essential for the first lead-free assembly to insure thermal requirements are met across the board. The diagram to the left shows the proper way to measure the heat applied to the balls in the grid array.

No wetting, due to low heat	Excessive temperature	Good thermal profile

The photo on the left shows balls, which have not undergone reflow due to insufficient heat. By measuring the temperature accurately at the ball site, this can be avoided. The temperature at the ball site had not seen 217 °C the melting point of SAC balls.

The photo in the center shows what happens when excessive temperature is seen by the BGA; in this case the temperature was measured at about 265 °C at the ball site.

The photo on the right shows the proper collapse of lead-free balls with the thermal profile properly set. The standoff distance may be higher with lead-free SAC due to its higher surface tension.

There are other reasons why lead-free reflow demonstrates poor wetting

• Solder paste activity level is too low
• Excessive preheat temperatures
• Too long a preheat
• Difficult to solder finishes
• Insufficient time above liquidus temperature
• Excessive oxidation of parts to be joined

Lead-free solder pastes require activation to be sustained beyond traditional tin-lead systems up to 217 °C and beyond for SAC alloys. Like traditional 63/37 no-clean pastes, such as ROLO types, the prevention of oxidation to parts and boards is critical. Flux classifications such as ROM1 may contain halides and are therefore better able to cope with oxides or difficult to solder parts.

Tin-Silver-Copper solders wet most metal surfaces more slowly and adequate times above the melting point of the solder is needed to achieve good wicking and solder spread. Normally the range is 60-90 seconds with peak temperatures from 235-245 °C. If soldering is jeopardized by oxidation of parts to be soldered, this can be verified using solderability test methods such as the wetting balance test.