Best Practices Reflow Profiling for Lead ... - KIC …

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. 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.

2 Smaller components are also more Engineer, Indium Corporation, is an SMTA-certified engineer and susceptible to tombstoning and defects related to solder paste slump. 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. 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.

3 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. 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.

4 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). 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.

5 Meaningless. Most important is the voiding, beading/balling, graping, and average ramp rate, which is C/. 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. 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.

6 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. 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. Unfortunately, many ovens lack controls for the cooling zone(s). One method to overcome this is to use the last heating zone(s), if the oven is large enough.

7 Figure 3 - Note that the maximum slope ( C/second) does not equate to the average ramp rate ( C/second). Water-Soluble vs. tight range (see Figure 2) for a specified time. This plateau in No-Clean Solder Pastes the Reflow profile allows the thermal gradient across the PCB At this point, it may be worth discussing the fact that there to equilibrate prior to Reflow . In this way, the entire assembly are distinct differences between water-soluble and no-clean sees nearly the same Reflow conditions (peak temperature and solder paste chemistries and their sensitivity to a soak or a time-above-liquidus) to form consistent solder bonds. Any long (total time in oven) type profile. For both types of solder potential thermal gradient increases as the physical size of paste, the activator portion of the flux chemistry removes the PCB and the diversity of component size increase, so for existing surface oxides on the mating surfaces, as well as the such large PCBs a soak profile is usually helpful to achieve powder particles within the solder paste itself.

8 Once removed, successful assembly. other flux chemistry constituents must protect the surfaces from re-oxidation. No-clean chemistries are generally rosin/. Soak profiles are also used to minimize voiding when resin-based materials. Rosins/resins make excellent oxide assembling such components as BGA (ball grid array), LGA barriers and protect the cleaned surfaces during Reflow (land grid array), SGA (solder grid array), and QFN (quad flat from re-oxidation. Water-soluble fluxes, on the other hand, pack, no leads), purging the solder paste of volatile materials, contain high molecular weight compounds, such as polymers, decreasing flux out-gassing, and diminishing the amount/size which are not as effective as rosins/resins at preventing re- of voids entrapped in the solder joint upon cooling. oxidation. Therefore, water-soluble chemistries do not hold up well in elongated Reflow profiles, such as soak profiles. If such Reflow profiles are used with a water-soluble solder paste, defects The actual Reflow of the solder alloy involves the creation of a such as de-wetting can occur.

9 Mechanical and electrical bond through the formation of tin- copper intermetallics. Powder Particle Size In forming optimum intermetallics, two critical parameters Particle size also affects the sensitivity of the solder paste to are involved in the Reflow phase: peak temperature and TAL the Reflow profile. As the particle size decreases, the overall (time-above-liquidus). The peak temperature is generally area of the surface exposed to oxidation increases. Increased 20-30 C above the liquidus temperature of the alloy and the surface oxide challenges the solder paste flux. Under these TAL is typically 30-90 seconds in order to form effective circumstances, a soak profile especially challenges the intermetallics. capability of the flux to provide an acceptable solder joint. Cooling The cooling phase determines the grain structure of the Troubleshooting solder joint. A fine grain structure provides the most reliable The focus of this paper is the effect of Reflow Profiling on mechanical bond.

10 To achieve this structure, a rapid cooling solder defects. Very often there are more effective and rate as the solder transitions from liquid (liquidus) to solid efficient methods of resolving solder defects than optimizing (solidus) is needed (the first ~50 C of cooling). The limiting the Reflow profile. For instance, it is widely accepted that 60- factor for the maximum cooling rate is the stress that is 70% of all solder defects can be traced back to the printer. From One Engineer To Another . 3. Indium Corporation Tech Paper With that said, there are certainly instances where fine tuning the Reflow profile will benefit. Also note that changing the Reflow profile can cause other solder defects. Figure 5 - Solder balling/beading. Figure 4 - An example of tombstoning. A maximum ramp rate from ambient to peak of C/second is recommended. If the solder paste is heated too quickly, Tombstoning solvents in the flux chemistry vaporize violently creating Tombstoning defects are due to unbalanced wetting forces, small explosions, leaving solder balls or paste aggregates generally found when assembling passive components (see (solder beads) isolated from the solder mass with no ability to Figure 4).