A reflow profile depends on multiple factors, including customer requirements, component characteristics and temperature limits, reflow oven performance, and the PCB itself. Most importantly, quality requirements should be defined according to how the profile will be used rather than by following a reference guide without considering the actual application.
Common reference documents include IPC-7530 for soldering process temperature profiles, IPC-9502 for PWB assembly soldering process guidelines, IPC/EIA J-STD-001 for soldered electrical and electronic assemblies, and IPC/JEDEC J-STD-020C for components ranging from small to very large packages.
In principle, a reflow profile is derived from data collected by recording temperatures at densely populated component locations during reflow. IPC-7050 provides guidance on the construction of reflow-profile testing tools and on various reflow temperature measurement techniques. In production, the same profile is often applied to multiple components. However, reflow data should be collected, analyzed, and documented at each independent process step when a component begins production. This provides process validation and preserves a record for future analysis.
1. Terms and Definitions
Alloy: Tin-Silver-Copper (Sn-Ag-Cu)
An alloy whose principal constituents are typically tin, silver, and copper, and which is used as a lead-free solder.
Alloy: Tin-Nickel-Copper (Sn-Ni-Cu)
An alloy whose principal lead-free solder constituents are typically tin and copper. It is suitable for wave soldering or reflow soldering.
Allowable Temperature
The temperature range within which an electronic circuit or component can perform its intended function.
Component
A single part, or a combination of parts, that performs its designed function when assembled together.
Cold Solder Joint
A solder connection with poor wetting. It is typically characterized by a light-gray appearance and voids caused by insufficient heat during the soldering process.
Disturbed Solder Joint
A solder connection in which the joined metals move while the solder is solidifying.
Dewetting
The partial or complete withdrawal of solder from a substrate that was initially wetted.
Flux
A chemically and physically active compound that promotes wetting of the base-metal surface. When heated, flux removes minor surface oxides and other films through the molten solder and protects the surface against reoxidation during soldering.
Flux Residue
Flux-related contamination present on or near the surface of a solder connection.
Head-in-Pillow Defect (HiP)
Also called a ball-in-socket defect, head-in-pillow is a soldering defect in which the solder paste wets the substrate but does not fully join the BGA solder ball. The resulting joint may provide electrical continuity but lacks sufficient mechanical strength. Such components can fail under relatively small mechanical or thermal stress. If the defect is not detected during functional testing, it can result in field failures.
Liquidus
The temperature at which the solder reaches a completely molten or liquid state.
Non-Wetting
A surface that is in contact with solder but resists accepting or melting into the solder.
Package
A container used to protect a circuit element or component and provide terminal connections to the rest of the circuit.
Peak Temperature
The peak temperature is obtained by placing a thermocouple at the location that must be monitored.
Component Cracking
Cracking that occurs during IC reflow, typically as a result of moisture absorption.
Reflow Profile
A graph showing the relationship between time and temperature as a PCB passes through a thermal process.
Ramp-Up
The portion of the profile in which the assembly temperature increases at a predetermined rate. The ramp rate must be controlled to prevent thermal damage to components.
Preheat and Soak Time
The time period used to ensure that the temperature becomes balanced throughout the assembly. During this part of the profile, solvents in the solder paste evaporate and the flux is activated to remove oxides. The soak time or preheat time extends from the minimum soak temperature, Ts Min, to the maximum soak temperature, Ts Max.
Solder Ball
A small spherical mass of solder, normally located around a solder joint or randomly distributed across the substrate.
Solder Bead at a Discrete Component
A solder ball located at the termination of a discrete component, typically a resistor or capacitor. It may also occur on large or small transistors. Its occurrence generally varies with the volume of solder paste and the reflow profile.
Time Above Liquidus
The time during which the solder alloy remains liquid. The assembly must remain in this state long enough to ensure that all components reflow properly.
Time at Peak
The time at which the component reaches its highest measured temperature.
Ts Max
The maximum soak temperature.
Ts Min
The minimum soak temperature.
Tombstone Effect
A soldering defect in which a component is pulled into a vertical or angled position, leaving one terminal poorly soldered or unsoldered.
Wetting
The spreading of solder over the base metal as intermetallic compounds form.
Solder Wicking
The movement of solder caused by the surface tension of the molten solder.
The definitions above are based on IPC-T-50, Terms and Definitions for Interconnecting and Packaging Electronic Circuits.
2. Development of Reflow Profiles
A reflow temperature profile is defined by the relationship between temperature and time during heating. Two basic profile types are commonly used: ramp-soak-spike (RSS) and ramp-to-spike (RTS).
The RTS profile is suitable for many applications in which improved soldering performance is required. The RSS profile is suitable for assemblies with greater thermal demands or applications in which the temperature difference, ΔT, must be reduced.
The profile should be cross-checked against the recommendations of the component manufacturer to ensure that none of the materials exceed their maximum temperature limits. Before defining a process and reflow profile, the thermal sensitivity of the components should be verified against supplier specifications or IPC-9602.
3. Ramp-Soak-Spike Profile (RSS)
Figure 1 | Ramp-soak-spike reflow temperature profile.
Recommended RSS Parameters
The RSS profile separates the initial ramp, the soak period, and the final reflow stage. This structure helps reduce temperature differences across the assembly and allows the solder paste flux to activate before the solder reaches the liquidus region.
4. Ramp-to-Spike Profile (RTS)
Figure 2 | Ramp-to-spike reflow temperature profile.
Recommended RTS Parameters
Unlike the RSS profile, the RTS profile moves more directly from the ramp stage toward the peak reflow temperature. This profile can be useful when the process requires a shorter thermal cycle, provided that the components, PCB, solder paste, and assembly configuration can tolerate the resulting thermal conditions.
5. Special Reflow Profiles
Improving Wetting
Wetting problems involving either components or the substrate can sometimes be improved by adjusting the reflow profile. When the wetting problem is widespread across the assembly, an appropriate profile adjustment may improve the process. When the problem is limited to a specific component, however, the more likely cause may be the plating or surface finish of the component or substrate.
A reflow profile can improve wetting characteristics, but it may also affect other devices on the assembly. A general approach for improving wetting is to slightly shorten the reflow cycle to approximately 3 minutes and increase the peak temperature by 10–15 °C. These changes must be evaluated against the thermal limits of all components and materials on the PCB.
Reducing Voids
By volume, solder paste contains approximately 50% flux. During reflow, some flux remains within the solder joint. Appropriate technical adjustments to the reflow profile can help reduce void formation, although other process variables may have a more significant effect.
- Ramp rate to the soak region: < 3 °C/s; typical value: 1–3 °C/s.
- Soak temperature: 150–200 °C for less than 90 s; typical value: 30 s.
- Time from 40 °C to peak temperature: typically 3–4.5 minutes.
- Time above liquidus: 30–90 s.
- Peak temperature: 230–260 °C.
- Typical peak temperature for SAC and REL alloys: 230–250 °C.
- Typical peak temperature for SN100C and low-silver alloys: 240–260 °C.
- Cooling rate: < ?4 °C/s.
Reducing Head-in-Pillow Defects in BGAs
The following reflow profile can help reduce head-in-pillow defects caused by deformation of the BGA package, particularly when the package material undergoes thermal distortion. Profile adjustment may require extending the time near the peak temperature so that the solder ball and solder paste can fully coalesce.
- Time from 40 °C to peak temperature: 4–4.5 minutes.
- Ramp rate to the soak region: 2–3 °C/s.
- Soak temperature range: 150–180 °C.
- Soak time: 30–90 s.
- Ramp rate from soak to peak: 1–1.5 °C/s maximum.
- Peak temperature: 230–260 °C.
- Typical peak temperature for SAC and REL alloys: 230–250 °C.
- Typical peak temperature for SN100C and low-silver alloys: 240–260 °C.
- Time above liquidus: > 60 s.
- HiP cooling rate from peak to 210 °C: 1–1.5 °C/s maximum.
- Cooling rate after 210 °C: < ?4 °C/s.
6. Potential Reflow-Profile Causes of Soldering Defects
The defect information above addresses potential causes associated with the reflow profile. Poor soldering can also result from numerous other process or material variables. For targeted process and profile assistance, the solder-paste supplier's technical support team should be consulted.
Operating outside the recommended profile range may be necessary when component temperature limits, fixtures, or a high density of components create unusual thermal conditions. A densely populated assembly may require a process window that differs from the standard profile recommendations because different locations on the PCB can heat at different rates.
When developing or modifying a profile, temperature measurements should be taken at representative locations, including thermally demanding components and areas with substantial copper or high component density. The measured profile should then be compared with component limits, solder-paste requirements, and the intended defect-control objective. A profile that improves wetting or reduces voids in one assembly may not be suitable for another assembly with different materials, package types, or thermal mass.
Reflow profiling is therefore a process-validation activity rather than a one-time oven setup. The selected profile must provide adequate flux activation, solder melting, wetting, and cooling while keeping the assembly within the thermal limits of its components and materials.
