![]() ![]() The picture shows 100% uniform diffusion of the tin/lead solder with the SAC 305 alloy. Hybrid Assembly Lead-free BGA IntermixingFigure 2 details the cross section of a lead-free plastic BGA using tin/lead solder paste. Once a profile has been proven, the identification and change control must be documented and communicated to all users. Given this window, tolerance factors must be considered for the process equipment, including measurement (thermocouples). A typical process window could be at a minimum 5☌ if the targeted maximum reflow temperature is 225☌ and the lowest tolerable case temperature (resulting from the BOM review) is 230☌. 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. Given the minimal process window, reflow profile verification and control become paramount. The process window for the maximum reflow and maximum component case temperatures becomes very narrow compared with an all tin/lead process. ![]() Reflow Profile ControlsWith the introduction of hybrid PCA manufacturing, process control of thermal reflow profiles becomes critical. Figure 2 below details the mixing required. To verify that the lead-free BGA has fully mixed, destructive analysis and cross sections will be required. Scrutinize voiding, wetting, solder quality, and overheating parameters for the remaining components on the assembly. 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. 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. The goal is for the lead-free BGA to fully mix the lead solder with the SAC sphere alloy as detailed in Figure 2. After completing the thermal profile, verify the readings to the solder paste supplier recommendations. Step 4: The completed BOM review of the case temperatures and BGA ball alloy will define the assembly's maximum allowable reflow temperatures. It is important to have a built-up assembly, given the thermal mass characteristics and limited process window. A reflow profile board or gold board is thermocoupled for direct temperature measurement of critical component joints. Step 3: The results from the assembly BOM review are known. For example, SAC305 and SAC405 alloys melt at similar temperatures however, a SAC105 BGA melts at 227☌, requiring an increase in the reflow profile. This BGA alloy distinction requires additional input into the reflow profile. Attention is required for the different types of lead-free BGA alloys (SAC305, SAC405, and SAC105). Various industry tools are available that complete this analysis automatically. The reflow profile creation will be gated by the component with the lowest maximum case temperature. Step 2: Bill of materials (BOM) analysis is critical. Step 1: Introduction of a lead-free BGA on a tin/lead assembly has been identified and communicated. The flowchart in Figure 1 provides a guideline for "leaded" hybrid assemblies using a five-step process approach.įigure 1. A traditional tin/lead reflow profile typically specifies a maximum reflow temperature of 210° to 217☌. Hybrid Assembly ProcessA lead-free plastic BGA with SAC 305 alloy requires collapse of the balls or spheres at approximately 217° to 220☌ to make a reliable solder joint. The term can also relate to a lead-free assembly that contains leaded components. 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. This transition has caused a new classification for printed circuit assemblies, politely defined as hybrid. Component manufacturers have obsoleted tin/lead or non-RoHS-complaint part numbers, with only RoHS-compliant offerings left. 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.įor companies using non-RoHS processes and continuing to specify leaded alloys, supply chain management has been, and continues to be, challenging. By Scott Mazur, Benchmark Electronics Inc. ![]()
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