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Step 7: Soldering
December 31, 1969 |Estimated reading time: 7 minutes
By Karen Walters
Developing and monitoring the solder paste manufacture's profile requirements is essential to ensuring that proper wetting occurs. Accordingly, attaching thermocouples to a printed circuit board (PCB) permits the operator to ensure that adequate temperatures are attained and that proper lead-to-pad interconnect junctions are formed as the board is processed through the reflow oven. Typical solder paste profile parameters include heating rates, soak times between temperatures, time required above the alloy melting point, peak temperatures and cooling rates.
Reflow Criteria Importance
Proper ramp rates will ensure that good solder joint grain structure will form and that potential thermal damage associated with coefficient of thermal expansion (CTE) mismatches between components and board materials will be minimized. Soak times permit components to achieve improved temperature equilibriums across a thermally challenging board before ramping to reflow peak temperatures. Time-above-liquidus parameters promote good wetting and ensure minimal embrittlement-causing intermetallic buildup within the solder interconnect. Component lead peak temperature requirements also are critical to ensure that the solder reflows properly and there is no thermal damage to the components or board. High-temperature exposure may cause internal damage to components, resulting in popcorning or molding compound delamination from the lead frame or die paddle, creating long-term reliability problems.
Thermocouple Attachment and Location
There are four typical methods of thermocouple attachment (Figure 1). High-temperature solder typically is the standard method, but this approach can be time consuming. An alternative - aluminum tape - has been favored in industry in recent years. It provides results similar to those of high-temperature solder while minimizing potential damage to the board and is environmentally friendly.
In the attachment process, it is essential that stress relief be provided to the thermocouple wires to prevent their lifting off from the solder joints. This can be done by routing the wires toward the opposite direction of travel in the oven and looping them through available drill holes on the board, or by mounting them to the PCB with epoxy or Kapton tape.
Thermocouple Location on the board is important for ensuring that all interconnections are soldered. Thus, the heaviest- and the lightest-mass components should feature thermocouple attachment. The coolest joints generally will be on large components near the center of the test vehicle while the hotter solder joints will be those of the smaller components located in less densely populated areas of the board - typically near the edge. Sensitive components, for which temperature or ramp rates are critical, also should have thermocouples attached.
For solder sphere array packages, the preferred location for temperature measurement is at the center and outer-edge solder spheres so that potential warpage conditions may be detected. Typically, a small hole is drilled from the underside through the laminate or substrate of the test vehicle and tangent to the center sphere of the targeted package. The hole then is filled with a high-temperature adhesive or cement. An alternative means is to route the thermocouple on the topside of the test vehicle through the package's outer solder spheres and approximately tangent to the center spheres. The thermocouple itself is anchored with high-temperature adhesive or ceramic at some strategic location on the test vehicle.
Thermocouple Types. For typical eutectic solder temperatures, a Type K thermocouple using Teflon-insulated 30 AWG wire is used for generating profiles. For lead-free soldering, a Type K thermocouple using glass-braid-insulated 30 AWG wire is recommended. This will ensure longer reliability of the wire at the higher lead-free temperatures. For temperature accuracy, standard limits of error are ñ2.2°C or 0.75 percent, whichever is greater. Where standard limits of error are unacceptable, special limits-of-error wire can be purchased whose accuracy is guaranteed above 0°C for ñ1.1°C or 0.4 percent, whichever is greater.
Data-logger Recommendations. Many different tools are available for collecting profile data. While some just collect information, others have profile prediction, profile optimization, oven control and statistical process control (SPC) features. The type of tool or data recorder to be selected is a function of the product, process and cost. The traveling type tool should follow the test vehicle at least by one oven zone in length, typically creating a 12 to 14" space between tool and vehicle. As the process window narrows, automated SPC will become critical for early detection of any process shifts.
The Reflow Oven
With increased board complexity, the incidence of solder reflow-related defects is increasing. For reflow ovens, the challenge becomes the ability to withstand higher temperatures, to obtain optimum uniform heat transfer and cooling, and to have an effective means for flux removal.
In radiation ovens, radiant heat transfer is a function of infrared (IR) wavelengths. Product emissivity can be an issue. Very dark surfaces have emissivities closer to 1, and hence, absorb or emit more energy than a shiny or light color object whose emissivity is closer to 0. When a product contains both high- and low-emissivity areas, it will heat at different rates and cause variances in product temperature. As a result, in a radiant oven, a larger temperature gradient typically will exist across a product.
Forced-convection heating has become the preferred means over IR reflow for soldering components to the board. This is because there are fewer factors that can affect heat transfer, resulting in better uniformity, ramp rates and process control. Convection ovens typically are heated electrically and use blower motors or fans that force hot air through orifice holes and onto the product. The temperature profile of the latter primarily depends on product mass, mass flow rate and process gas temperature. Forced-air convection ovens are more efficient in the temperature range in which solder reflow is accomplished and, thus, yield tighter process control.
It is key to realize that in forced-air convection ovens, temperature setpoints increase within the oven as the convection rate decreases owing to the density of the gas, prompting the decline of heat-transfer efficiency. In response, some of today's forced-air convection ovens have features that counter this effect. One example is closed-loop control in multiple zones to maintain constant convection rates independent of temperature, voltage and altitude variations.
Component Temperature Equilibrium
Another point to consider: Soldering can be problematic for larger products with uneven mass distribution that use the preferred optimized reflow profile (Figure 2). In the past, a soak was introduced into the profile to achieve component temperature equilibrium before elevating their leads to peak temperature: The optimized profile is a straight ramp to peak temperature. This profile is used to prevent flux activator burn-off before reflow. As a result, better wetting and less solder defects and thermal shocks occur. Additionally, this is the preferred profile for lead-free soldering where the new alloys are more difficult to wet.
An oven must produce tight thermal uniformity to meet the optimized profile requirements. Hence, closed-loop forced-air convection, which ensures constant convection rates independent of process temperatures, is a key benefit of today's reflow soldering criteria. However, not all forced-air convection ovens are equal. While all units accomplish some level of heat transfer via radiation - be it direct or indirect - the ovens that possess the lowest levels of radiant heat transfer generally yield products with the lowest temperature gradient.
Other oven considerations during solder reflow are controlled cooling capabilities, maximum operating temperatures and a flux-management system. The oven should be capable of providing flexible cooling rates based on product requirements, and should be constructed with materials that can maintain the higher operating temperatures of lead-free solders. Lastly, a flux-removal system preserves the oven's internal cleanliness and performance.Oven Atmosphere Whether to use nitrogen as an atmosphere-inerting agent in the oven or forced air depends on the solder paste manufacture's flux chemistry makeup and board assembly attributes. Nitrogen protects the metal surfaces from oxidation during heating and ensures proper flux activation. Comparing the wetting force for different fluxes in air and nitrogen reveals nitrogen coverage can improve the process. N2 also enhances solder joint quality and esthetics, reduces the residue of some fluxes, and minimizes in-circuit test failures. For boards whose surface finishes feature organic solderability protection (OSP), the best yields are achieved when soldered in nitrogen-infused atmospheres. However, the actual oven oxygen content allowed should be evaluated before nitrogen implementation to evaluate the gas' cost vs. benefit factors. Ultimately, reflowed product quality is only as good as the investment in proper thermocouple instrumentation, data recording equipment, the environment and the reflow oven itself.
REFERENCES
- C. Sinohui, "A Comparison of Methods for Attaching Thermocouples to PCBs for Thermal Profiling," KIC Thermal Profiling, NEPCON West, 1999.
- L. Ning-Cheng, "Optimizing Reflow Profiles Via Defect Mechanisms Analysis," Indium Corp. of America, Utica, N.Y.
- P. Zarrow, A Guide to Reflow Profiling, ITM Consulting, Durham, N.H.
- A Code of Practice for Thermal Profiling Electronic Assemblies, National Physical Laboratory.
- J.P. Holman, Heat Transfer, McGraw-Hill Book Co., New York, 1986.
- S. Kasturi, "Forced Convection: the Key to SMT Reflow," Digital Equipment Corp., Augusta, Maine.
- Theriault, Blostein and Rahn, "Nitrogen and Soldering: Reviewing the Issue of Inerting," SMT's Guide to Printing and Soldering, September 2000.
- Tenya and Adams, "Thermal Effect on Components," SMT Magazine, November 2000.
- Shina, Walters, Biocca, Skidmore, Pinsky, Provencal and Abbot, "Selecting Material and Process Parameters for Lead-free Soldering Using Design of Experiments," APEX 2001, San Diego.
Karen Walters may be contacted at BTU International, 23 Esquire Rd., North Billerica, MA 01862; (978) 667-4111, Ext. 211.
Figure 1. Methods of thermocouple attachment for monitoring the reflow soldering process include the solder attachment (a), aluminum taping (b), attachment via Kapton tape (c) and conductive adhesive use (d).
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Figure 2. Chart compares conventional and optimized profiles. The latter is a straight ramp to peak temperature to prevent flux burn-off, improve wetting and preventthermal shock.
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