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by Lee Whiteman
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Figure 1. Example of Contact Angle. |
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Lead Free soldering is no longer an esoteric concept. In Europe, the Waste Electronic and Electrical Equipment (WEEE) and the Restriction of Hazardous Substances (RoHS) Directives stipulate that consumer electronic equipment sold must be lead free by July 1, 2006. Members of the European Union have the option to accelerate this deadline. For example, Germany is considering banning lead from electronics by August, 2004. In Asia, the Japanese Ministry of Industry and Trade Institute (MITI) has stipulated that lead usage be reduced by 67% by December 31, 2005. As a result, electronics manufacturers have taken the initiative to start building lead free soldered hardware. There are several technical issues associated with implementing the lead (Pb) free soldering process. In general, lead free solders require higher processing temperatures than their tin-lead (SnPb) counterparts
(Table 1).
Pb free solders do not wet as well as SnPb. Typically, SnPb will have contact angles of between 4° to 6° on a copper surface. The Pb free solder alloy, Tin (Sn) Silver (Ag) Copper (Cu), generally has contact angles ranging from 6° to 12°. Soldering in an inert atmosphere will improve the contact wetting angle by 1° to 3°
The processing variables for Pb free soldering are different from SnPb. Despite these differences, it is feasible to manufacture hardware that meets IPC-A-610 Class 3 requirements for high reliability hardware. It has been shown by the Lead Free Components Focus Group and NCMS that Pb free solders can pass aerospace thermal cycle testing from -55°C to 125°C.
HAND SOLDERING
Due to the higher soldering temperatures required for Pb free solders, the solder tip temperature has to be set higher. For example, with the solder alloys soldered, ACI found that the solder tip temperature had to be set at 343°C for Pb free solders, as opposed for 315°C for SnPb. It was discovered that a longer dwell time, the time the soldering iron was in contact with the hardware, was required to promote adequate heat transfer during the soldering process. It was ACI's experience that the soldering iron had to be removed quicker for Pb free than for SnPb. Icicles will be created if the soldering iron is removed too slowly. The size and frequency of solder icicles is dependent upon the purity of the alloy used and the soldering iron temperature setting
(Figure 2). Also, pure metals have a narrow freezing range.
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The higher soldering temperature requires that the soldering iron must remain clean and coated with the solder alloy. Pb free solders are more sensitive to the effects of a dirty soldering iron. The higher soldering temperatures can result in the soldering iron tip becoming oxidized if not cleaned and coated. The resulting solder joint will have a grainy dull finish. Depending upon the soldering operation, a more active solder flux may be required to promote wetting. Using a more active solder flux may require a more aggressive cleaning operation. WAVE SOLDERING ACI wave soldered hardware for the Lead Free Components Focus Group. There were several process differences found when compared to SnPb wave soldering operations:
- The preheat temperature was much higher for Pb free solders (165°C) than for SnPb (120°C).
- Pb free solders create more solder dross when soldered in air. Nitrogen (N2) reduces the amount of solder dross used.
- Due to the poor wetting of Pb free solders, N2 and more active solder flux will have to be used to promote better solderability.
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 It is documented that soldering in an inert atmosphere will improve solder wetting and reduce solder residues. Using a more active solder flux may require more aggressive cleaning. The resulting solder joint will have a grainy dull finish. The solder joint will have more voids if the profile is not properly tuned. It has been observed on SnBi alloys that there is a higher rate of lifted pads. This is caused by the cooling rate being too severe (Figure 3).
There is evidence that special tooling and tooling finishes may  be required for Pb free solders. The high Sn content may damage sections of the solder pot (Figure 4). Companies are now beginning to coat solder pot fixtures with ceramic and teflon to reduce this damage. It is recommended that a wave solder pot not run continuously. To extend the life of the solder pot, the pot should not remain idle for an extended period of time. The solder pot should be on a rigid preventative maintenance operation, concentrating on the solder pot's impeller and wave solder fixtures.
SMT MANUFACTURING
ACI found that for screen printing operations and component placement operations, SnPb and Pb free solder pastes were equivalent. ACI technicians found Pb free solder paste easy to use. There were no changes in the stencil design and screen printing parameters to achieve good solder paste application. From a component placement perspective, the Pb free solder pastes used had equivalent tackiness and green strength to their SnPb counterparts. For reflow soldering, the differences between Pb free solders and SnPb solders are wider. Typically, the peak reflow soldering temperature for Pb free solders can range between 240°C and 260°C. The thermal profile will be dependent upon the Pb free solder alloy used and the vendor. For example, ACI discovered that for the same SnAgCu alloy, two different vendors had each recommended different reflow soldering profiles and peak reflow soldering temperatures. It is recommended that soldering be performed in an inert atmosphere, such as N2. This will improve solder wetting and reduce solder residues. With respect to the equipment, depending upon the vintage of the reflow soldering ovens used, current equipment can reach the reflow soldering temperatures required to process Pb free soldered hardware.
The equipment settings will be dictated by:
The solder alloy used
The hardware's thermal mass |
To reach the higher reflow soldering temperatures, it is possible to reduce the oven's belt speed. The slower belt speed will reduce productivity. Due to the higher temperatures, more preventative maintenance - specifically belt lubrication and panel maintenance - will be required. New ovens on the market are capable of supporting Pb free soldering.
INSPECTION
Pb free solder joints have a different appearance to SnPb solder joints (Figure 5). The Target Inspection Criteria for solder joints, based on IPC-A-610, is that the solder joint will have a bright shiny appearance, a smooth surface finish, and good wetting coverage on the pad and lead.
Pb free solders have a dull grainy surface finish. As previously\ indicated, Pb free solders do not wet as well as SnPb. IPC A-610C, Paragraph 6.1, was included to take into account the results from using Pb free solders. It reaches across Class 1, Class 2, and Class 3 packaging requirements. It allows for solder alloys which produce:
- Dull matte surface - Gray color - Grainy appearance -
- Considered normal for the materials or processes involved
These changes take into account the differences when using Pb free solders. However, operators which are familiar with the output on SnPb soldering, would have to be recalibrated when inspecting Pb free solders.
FUTURE ACI LEAD FREE SOLDERING PROJECTS
The EMPF will continue to perform research and development in Pb soldering. A Pb solderability analysis will be performed to quantify what level of contamination or oxidation prevents Pb solders from being soldered. Pb contamination will be investigated to determine what level of Pb contamination reduces Pb free solder joint reliability. Due to the high Sn content of Pb free solders and finishes, Sn whiskers is becoming a major concern. ACI hopes to correlate Sn whisker growth to the environment, and board and component finishes. ACI’s EMPF offers a two-day Pb free soldering course.This course uses various Pb free solders within ACI's Demonstration Factory. Participants are encouraged to bring samples of their hardware for soldering. If there are any questions, please feel free to contact the Electronics Manufacturing Learning Center Registrar at (610) 362-1320.
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