Combining Tin/Lead and Lead-free: A 5 Step Hybrid Manufacturing Process

Estimated reading time: 5 minutes

By Scott Mazur, Benchmark Electronics Inc.

Three years have passed since the European Union (EU) adopted the Restriction of Hazardous Substances (RoHS) regulation, and the transition to RoHS compliance has impacted the entire electronics industry.

For companies using non-RoHS processes and continuing to specify leaded alloys, supply chain management has been, and continues to be, challenging. Component manufacturers have obsoleted tin/lead or non-RoHS-complaint part numbers, with only RoHS-compliant offerings left.

This transition has caused a new classification for printed circuit assemblies, politely defined as hybrid. The term hybrid within this article refers to a printed circuit assembly that uses tin/lead solder and contains lead-free plastic ball grid arrays. The term can also relate to a lead-free assembly that contains leaded components.

Hybrid Assembly ProcessA lead-free plastic BGA with SAC 305 alloy requires collapse of the balls or spheres at approximately 217° to 220°C to make a reliable solder joint. A traditional tin/lead reflow profile typically specifies a maximum reflow temperature of 210° to 217°C. The flowchart in Figure 1 provides a guideline for "leaded" hybrid assemblies using a five-step process approach.

Figure 1. Hybrid assembly five-step flowchart.

Step 1: Introduction of a lead-free BGA on a tin/lead assembly has been identified and communicated.

Step 2: Bill of materials (BOM) analysis is critical. The reflow profile creation will be gated by the component with the lowest maximum case temperature. Various industry tools are available that complete this analysis automatically. Attention is required for the different types of lead-free BGA alloys (SAC305, SAC405, and SAC105). This BGA alloy distinction requires additional input into the reflow profile. For example, SAC305 and SAC405 alloys melt at similar temperatures; however, a SAC105 BGA melts at 227°C, requiring an increase in the reflow profile.

Step 3: The results from the assembly BOM review are known. A reflow profile board or gold board is thermocoupled for direct temperature measurement of critical component joints. It is important to have a built-up assembly, given the thermal mass characteristics and limited process window.

Step 4: The completed BOM review of the case temperatures and BGA ball alloy will define the assembly's maximum allowable reflow temperatures. After completing the thermal profile, verify the readings to the solder paste supplier recommendations. The goal is for the lead-free BGA to fully mix the lead solder with the SAC sphere alloy as detailed in Figure 2. Special attention is needed to verify the peak reflow temperature does not exceed the individual component specifications, which could result in either immediate or latent component failures.

Step 5: Perform a first article visual and X-ray inspection to confirm that the known lead-free BGA balls have collapsed and wetted properly with acceptable or no voiding. Scrutinize voiding, wetting, solder quality, and overheating parameters for the remaining components on the assembly. To verify that the lead-free BGA has fully mixed, destructive analysis and cross sections will be required. Figure 2 below details the mixing required.

Reflow Profile ControlsWith the introduction of hybrid PCA manufacturing, process control of thermal reflow profiles becomes critical. The process window for the maximum reflow and maximum component case temperatures becomes very narrow compared with an all tin/lead process. Given the minimal process window, reflow profile verification and control become paramount. The verification should be completed during creation and revision to confirm the process parameters (maximum temperatures, ramp, etc) are acceptable to internal procedures and the solder paste manufacturer's recommendation. A typical process window could be at a minimum 5°C if the targeted maximum reflow temperature is 225°C and the lowest tolerable case temperature (resulting from the BOM review) is 230°C. Given this window, tolerance factors must be considered for the process equipment, including measurement (thermocouples). Once a profile has been proven, the identification and change control must be documented and communicated to all users.

Hybrid Assembly Lead-free BGA IntermixingFigure 2 details the cross section of a lead-free plastic BGA using tin/lead solder paste. The picture shows 100% uniform diffusion of the tin/lead solder with the SAC 305 alloy. The black lines are the lead mixing, which is uniform from the package to the PCB pad. The mixing resulted from a maximum reflow temperature of 225°C. The key result is uniform diffusion and collapse of the plastic lead-free BGA when using tin/lead solder paste. Figure 2. Hybrid BGA solder joint.

ConclusionWith the electronics industry movement to lead-free components, hybrid manufacturing may be a necessary solution if a non-RoHS assembly uses tin/lead solder and lead-free BGAs are being introduced to the assembly and process. Diligence and process controls are required to verify the acceptable case temperatures for all components on the BOM, reflow profile parameters, and appropriate alloy mixing. Additional testing and failure analysis (FA) is also recommended to verify the reliability of the design and overall assembly's technology.

References:1. "Green Electronics Design and Manufacturing Book," McGraw Hill Publishing Authored by Sammy Shina, Contributing Author, Bob Farrell & Scott Mazur (Chapter 6)

Acknowledgement:BGA Mixing picture was courtesy of Paul Bodmer and Bruce Tostevin, Benchmark Electronics Inc., Hudson, NH division.

Scott Mazur, manufacturing staff engineer, Benchmark Electronics Inc., Hudson, NH Division, may be contacted at scott.mazur@bench.com.

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