Both pastes were run through a reflow oven at 180 °C. Paste B has better hot slump properties than Paste A and would less likely cause bridges, solder balls or mid-chip balling. For fine pitch components it is critical to select a lead-free paste with a heat stable gelling agent.
Poor wetting of terminations and pads
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.
Poor solderability, insufficient wetting, poor wicking of solder, and large contactangles 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.
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 and the main causes are summarized below:
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.
Voids in lead-free joints and BGA’s
It must also be stated that smaller voids can in cases increase reliability by changing the crack pattern. Studies have shown that there is no reduction in reliability when voids are present to up to 25% by volume in the joint. Voids can act as stress relievers, due in part to the compressive nature of air pockets.
This is documented in the technical paper, Voiding: Occurrence and Reliability Issues with Lead-free, by Martin Wickham of the National Physical Laboratory.
Some causes of voids in joints are summarized below:
Lead-free alloys such as SAC alloys have slight higher surface tensions when compared to 63/37. It is important to select a solder paste which has a flux chemistry designed for the higher preheats and peak temperatures. Choosing a solder paste, which does not contain resins and activators which decompose at these higher temperatures is the primary factor in void reduction. Good solder paste manufacturers are designing flux systems for lead-free alloys. The voiding potential information is often available for use during the paste selection process.
Optimizing the reflow profile as to remove any volatiles by extending the preheat times and increasing the time above liquidus will also help in reducing void entrapment. Insuring components and boards are free of moisture and plating contaminants will also help to reduce voids. It has been shown that copper OSP tends to produce slightly higher volume of voids when compared to Ni/Au and silver immersion, which produce much less.
In some cases joint geometries are contributors. Components such as leadless chip carries or large flat surfaces, perpendicular to the board will prevent out-gassing during the soldering process; this results in void increases. Solder flux by-products both
liquid and gases, will have to slowly make there way upwards. Component geometries which prevent the proper upward flow, will usually result in an increase in voids.Tombstoning defects with lead-free
Lead-free may increase the uplifting of smaller components. This is due in part to the reduced wetting behavior of lead-free alloys. Component placement is more important with lead-free alloys since less centering will occur during reflow. This can increase the incidence of tombstones.
SAC305 tends to reduce tombstones, this alloy has a concentration of 96.5 Tin, 3.0 Silver and 0.5 Copper and has melting range of 217-220 ° C. Because of the small pasty range the component prone to tombstone is tacked by the initial melting phase of the alloy.
A solder paste, which exhibits excessive out-gassing during the initial stages of the melting of the solder powder, will also increase tombstone defects. The paste manufacturer must carefully choose resins and solvents which do not decompose or vaporize at the melting point of the alloy.De-wetting with lead-free
De-wetting is often due to a lack of flux activity. This behavior rarely occurs with water-washable type pastes since these pastes are highly activated. Lower activity solder pastes in the category of ROLO, halide free no-cleans pastes tend to create this on more difficult finishes such bare copper OSP or on Ni-Au where the nickel base metal, may have experienced oxidation or plating contamination.
Below are test coupons on which SAC no-clean paste was applied to two surfaces.
The test coupons were then reflowed in air using the manufacturer’s recommended thermal profile. The one on the right shows de-wetting while the one on the right exhibits good wetting. The pooling of the solder was due to the base metal being difficult to solder to. The molten solder initial spread across the surface but not a good enough intermetallic bond was formed, resulting in surface tension pulling the solder away.
Ways to reduce or prevent de-wetting with lead-free SMT are:
Excessive dullness and surface effects with lead-free
Below are two photos. The one on the left is 63/37, while the other shows joints done with SAC305 alloy.
Lower peak temperatures and lower times above liquidus will reduce both intermetallic growth but also increase the overall brightness of the solder joints.
Proper training will be required when transitioning to lead-free assembly. Operators will need to be given quality acceptance criteria for solder joints that will look quite different from traditional leaded systems.
About the author:
For further information please contact Peter Biocca at Kester, telephone 972.390.1197 or via email at email@example.com
OK International, Photos of BGA Optical Inspection and Cosmetic Joint Comparisons.
Bob Willis U.K., Photos BGA with SAC alloy.
Gintic Manufacturing Consortium, Singapore. Lead-free Report.
Kester DesPlaines , Illinois , Applications Laboratory, Photos Paste Slump, Spread Tests.
Voiding: Occurrence and Reliability Issues with Lead-free, Martin Wickham, National Physical Laboratory, U.K.
Lead-free Electronics, 2004 Edition, Sanka Ganesan; Michael Pecht, Calce Press.