Soldering Atmosphere’s Impact on Solder Joint Formation Print E-mail
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Written by Ursula Marquez de Tino   
Friday, 29 February 2008 19:00

A Flextronics-Vitronics study finds the delivery method’s impact is minimal.

Wave Soldering The 2007 iNEMI Roadmap identifies an increase in the use of nitrogen in reflow and wave soldering processes because of the lower wetting forces of Pb-free solders. While flux chemistries are being developed and improved to promote wetting of Pb-free alloys, it will take many iterations and time to achieve the desired results. The iNEMI Roadmap emphasizes nitrogen consumption reductions through alternative low-volume distribution methods. In combination with advanced flux chemistries, this would yield the desired quality and reduce operation costs.

To provide clarity to this industry gap, Flextronics and Vitronics Soltec jointly investigated soldering atmosphere’s impact on assembly yields. The question of immediate importance is what purity level and delivery method can be employed without degrading solder joint yield. To address this, the investigation defined two gas delivery techniques at five selected atmospheres specified by the O2 ppm level for reflow assembly of a test PCB. The two nitrogen delivery methods studied were distribution of nitrogen source gas only in the reflow zones, reflow only (RO), and distribution of nitrogen source gas throughout the length of the heated and cooled tunnel, full tunnel (FT). The atmosphere purity for both delivery methods was initiated at a low of 25 ppm (O2) and then increased to 500 ppm, 750 ppm, 1200 ppm and 2500 ppm before concluding in air atmosphere. Yield was determined by quantitatively characterizing the solder’s ability to completely fill vias during a subsequent wave process. The assembly matrix consisted of exposing the boards to two Pb-free reflow processes at each of the defined purity levels and distribution methods. The rationale for specifying hole fill as the limiting quality factor was to identify and expose PCB material and pad finish to the worst possible soldering conditions. Characterization of the assemblies and respective through-hole penetration was performed by measuring the solder rise with an Agilent 5DX programmed to take measurements at each 10% of the via from 100% to 20%.

The 16-layer, 0.125"-thick test PCB was FR-4 (Tg of 170°C, Td of 350°C) with Entek Plus HT copper OSP pads. The surface finish was selected for its high industry use and sensitivity or degradation when exposed to elevated temperatures and varying oxygen environments. All through-hole components used in the assembly were rated for Pb-free processing and had Pb-free finish. The components included three 64-pin connectors with gold finish, three plastic dual-inline packages (DIP) with 16 pins and 100% tin matte finish, and 25 axial resistors with 100% tin finish. For all PTH components, except DIPs, the leads were mechanically trimmed to achieve lead protrusions of approximately 0.140".

The flux and solder alloy were resin alcohol base no-clean flux with 7% solids content and SAC 305, respectively. Boards were subjected to two Pb-free reflow profiles based on the paste manufacturer’s specifications.

The results and data analysis provide some intuitive and surprising conclusions. For most types of mid- to high-level complexity employment of a soldering atmosphere where the delivery method is reflow only and atmosphere purity is 2500 ppm, an O2 soldering atmosphere will yield acceptable joints per IPC Class 3. In these cases, the through-hole penetration observed was just under 100%, but clearly visible while viewing the board. For “simple” boards, air atmosphere is suitable. The impact of delivery method was minimal at best, which was the surprising result (Figure 1). Use of high purity atmospheres in the early part of the reflow process exerts a minor effect on through-hole penetration for those simple- to mid-level complexity boards. While flux was not the focus of the experiment, flux did exert a significant effect and may influence desired results.

Image

In conclusion, this work sheds light on nitrogen use and its impact on solder joint formation. When developing a soldering process, whether to use nitrogen depends on the materials selected, especially flux, board complexity, and defect definition and characterization. Characterizing the relationship between atmosphere purity, delivery method, nitrogen consumption and preventive maintenance is also important to completely determine atmosphere’s impact on solder joint formation.

Ursula Marquez de Tino
is a process and research engineer for Vitronics Soltec (vitronics.com); This e-mail address is being protected from spambots. You need JavaScript enabled to view it .

 

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