Best Practices Reflow Profiling for Lead-Free SMT Assembly ...

1 Indium Corporation Tech Paper best Practices Reflow Profiling for Lead-Free SMT Assembly Authored by: Ed Briggs and Ronald C. Lasky, , PE. ABSTRACT Ed briggs The combination of higher Lead-Free process temperatures, smaller print deposits, and temperature restraints on electrical components has created difficult challenges in optimizing the Reflow process. Not only are the electronic components and the PWB at risk, but the ability to achieve a robust solder joint becomes difficult, especially if the PCB is thermally massive.

2 In addition, the constant miniaturization of electronic components, hence smaller solder paste deposits, may require the use of smaller particle-sized powders. Both the small solder paste deposits and small particle size result in a large surface area-to- volume ratio that challenges the solder paste's flux to effectively perform its fluxing action. The possible resulting surface oxidation can lead to voiding, Ed Briggs, Technical Support graping, head-in-pillow, and other defects. Smaller components are also more Engineer, Indium Corporation, is an SMTA-certified engineer and susceptible to tombstoning and defects related to solder paste slump.

3 Has earned a Six-Sigma Green Belt Certification from Dartmouth College This paper is a summary of best Practices in optimizing the Reflow process to for demonstrated proficiency in meet these challenges of higher Reflow temperatures, smaller print deposits, developing and executing design decreased powder particle size, and their affect on the Reflow process. It also experiments to support continuous discusses trouble-shooting of the most common defects in Lead-Free Reflow , such process improvement. as tombstoning, solder beading/balling, residue discoloration, voiding, graping, email: and head-in-pillow.

4 Full biography: corporate/ Key words: solder defects, Reflow profile, tombstone, solder beads, solder balls, voids, head-in-pillow, graping. A QR (quick response) code contains encoded data. INTRODUCTION can restrict the peak temperature that When scanned with a smart phone's The introduction of higher Lead-Free the process can use, making it difficult camera (via a QR reader application), to achieve a robust solder joint, it will take you to a specific URL or process temperatures and a reduction text message. in solder paste deposit volumes especially if the PCB is thermally require narrower process windows to massive.

5 Decreased pad size might Download article optimize the Reflow profile. Not only also require the solder paste to have Share with are the electronics components and the solder powder with smaller particle a friend PWB at risk due to the higher Reflow diameters. Both the small solder paste temperatures associated with Lead-Free deposits and smaller particle size result processes, the components themselves in a large surface area-to-volume ratio Form no. 98675. From One Engineer To Another . 1. Indium Corporation Tech Paper Ronald C.

6 Lasky, PhD, PE. Ronald C. Lasky, , Senior Technologist, Indium Corporation, holder Figure 1 - Ramp-to-peak Reflow profile. of the prestigious SMTA Founder's Award, is a world-renown process expert and an Instructional Professor at Dartmouth College. He has over 25. years of experience in electronics and optoelectronic packaging and Assembly . He has authored or edited five books and numerous technical papers and holds several patent disclosures. email: Full biography: corporate/bio/. between heating zones ( T of <40 C is recommended between heating zones).

7 Figure 2 - Ramp/soak/spike Reflow profile. Typically the instantaneous ramp rate that challenges the solder paste's flux to Profile Stages will vary as seen in Figure 3. Hence, the effectively perform its fluxing action. The SMT Reflow profile can be broken maximum ramp rate of C/second, The resulting surface oxide can lead down into 4 phases or regions: preheat, reported in Figure 3, is somewhat to a number of solder defects such as pre- Reflow , Reflow , and cooling. meaningless. Most important is the voiding, beading/balling, graping, and average ramp rate, which is C/.

8 Head-in-pillow. This paper will discuss Preheat second. techniques to optimize the Reflow The preheat phase preconditions the profile to minimize such defects. PCB Assembly prior to actual Reflow , Pre- Reflow removing flux volatiles and reducing At the pre- Reflow phase, the flux Types of Profiles thermal shock to the PCB Assembly . activator (or activator package). The two most common types of Reflow removes any existing surface oxide profiles are the ramp-to-peak profile, The ramp rate is the slope of temperature from the component leads and PWB.

9 Also called ramp-to-spike or tent profile versus time for the heating portion pad finishes, as well as any oxide on (see Figure 1) and the soak profile, or of the Reflow profile. It originates at the powder particles within the solder ramp/soak/spike profile (see Figure 2). ambient temperature and ends at the paste itself, preparing the surfaces to be A ramp-to-peak profile is a linear ramp peak temperature. The rate is defined primarily in the preheat phase. A ramp joined during Reflow . to the peak (max) temperature. A soak profile displays some plateau within rate of C/second is normal A temperature soak, if incorporated, a limited temperature range, before the and is largely affected by the Reflow is normally found in this phase.

10 The alloy reflows. oven belt speed and the -T (delta-T) soak temperature is controlled within a From One Engineer To Another . 2. Indium Corporation Tech Paper exerted on the solder joint if the rate is too fast. This thermal stress, depending on the differences in CTE (coefficient of thermal expansion) of the joining surfaces, can fracture or tear the solder joint. The greater the difference in CTE of the joining materials and the cooling rate, the greater the thermal stress generated. A cooling rate of ~4 C/second is normal.