Author(s): Hunter Pullishy, Sandy Yimbo, and Jose Pineda
August 1, 2022 at 2:00:00 p.m.
Abstract
In the electronics manufacturing industry, head-in-pillow (HIP) defects are one of the most common issues that affect circuit boards containing BGA/LGA (Ball Grid Array/Land Grid Array) packages. These defects can result in costly repairs and greatly reduce a component’s lifespan. HIP defects are compromised solder joints that are often attributed to undesired environmental factors during the reflow process. These factors include reflow in an oxygen-filled environment, exposure to temperatures surpassing a component’s thermal limit, and uneven thermal distribution across the PCB (Printed Circuit Board). The exploration of innovative reflow processes has led to the renewed adoption of Vapor Phase Soldering within the electronics manufacturing industry. Vapor Phase Soldering introduces an oxygen-free environment and a unique heating process that could address the cause of HIP defects. Collecting images and data from a reflowed BGA/LGA hybrid connector in a convection oven and then using vapor phase technology for a rework, we look to investigate if the Vapor Phase Reflow process addresses this defect. When analyzing the data we found a noticeable improvement in solder quality as well as increased coplanarity after the Vapor Phase rework. These findings offer preliminary support for the benefits that are offered when reflowing PCBAs (Printed Circuit Board Assembly’s) using the Vapor Phase Soldering method.
Introduction
BGA/LGA hybrid connectors function much like the human body’s nervous system. They allow the brain, known as a microprocessor, to interface with all other components on the PCBA. The sensitive digital cortex has a low tolerance for faults and a PCBA without its brain is little more than some copper and silicon. One common fault that threatens a system’s integrity is the head-in-pillow (HIP) defect. These defects have become increasingly common with the adoption of lead-free alloys in BGA-style components. Although the development of HIP defects can result in immediate intermittent failure of a PCBA, the more common outcome is a failure in the field due to moderate or thermal stress. The defects tend to form during the reflow process and a few factors that often contribute to their formation include: exceeding a component’s thermal limitations, the unequal distribution of heat during reflow, and exposing a component to an oxygenated environment. This non-ideal environment, and the subsequent development of HIP defects, has plagued the industry for many years. Before the late 1980s the preferred reflow method was Vapor Phase Soldering due to its enhanced heat transfer capabilities [1]. However, because of speculation over its negative environmental impact, Vapor Phase technologies were abandoned in mainstream electronics manufacturing. Modern innovations in the Vapor Phase Reflow process have resulted in the adoption of PFPEs (perfluoropolyethers) which have a reduced environmental impact. This has caused a resurgence in the use of Vapor Phase reflow in many industries, especially those with low tolerance for electrical failures. The changes in the reflow process between convection and vapor phase put into question which process would be more favorable regarding HIP defects. Research conducted by Leicht and Thumm indicated the reflow environment observed when using Vapor Phase Soldering reduced the conditions that cause HIP defects [2]. To investigate the impact of changing the reflow environment, a comparison was made between convection and vapor phase processes. The discrepancy between the two processes supports the conclusion that using Vapor Phase technology reduces head-in pillow defects, addressing its key causes and making it an effective countermeasure.
Fig. 1. Example of a Head-In-Pillow defect where image a) is the optical micrograph of a solder joint. This defect is likely caused by an oxide layer forming between the pad and lead. Image b) is the side view of the HIP defect effecting a BGA solder ball [3].
What is a Head-In-Pillow Defect?
As illustrated in Figure 1, head-in-pillow defects are mechanically weakened solder joints. These defects are most commonly found on BGA/LGA style packages. They often retain electrical integrity which allows them to pass functional tests, yet they still result in in-field failures [4]–[6]. As previously mentioned, these failures are due to mechanical or thermal stresses that are exerted on the defective component. Due to the nature of BGA packages these defects can be costly.
How Does a Head-In-Pillow Defect Occur
There is a multitude of factors that can result in head-in-pillow defects. One of the primary causes is the occurrence of a common solder defect known as ”poor wetting” [4], [5]. This issue is often the result of oxidation during the soldering process. Oxidation is the chemical reaction between oxygen molecules and exposed metal that results in the formation of oxide layers. These layers cause imperfections in the solder joints which lead to the aforementioned defects. Traditionally, flux is used to break down these oxide layers, albeit without complete effectiveness.
Fig. 2. Diagram representing one cause of HIP defects. As the solder melts, the land pad and BGA ball disengage. After the solder starts cooling they make contact again, yet do not form a bond [7].
Another factor that can cause a HIP defect is warpage. Warpage is the deformation of either the components or the PCB [5], [8]. Different materials will expand at different rates when exposed to high temperatures. This is further exaggerated if the heat is disproportionately distributed across the surface of the material. As PCBs and components are soldered they undergo a heating cycle which uses different temperature zones to gradually heat the PCBA. This can lead to uneven thermal distribution, as one end of the PCBA is heated before the other, subsequently leading to PCB warpage. The different temperature zones require a higher peak temperature as the PCB must be held above the solder paste’s melting point for up to 30 seconds. This is often achieved by increasing the maximum applied temperature up to 35 degrees Celsius, as per the solder paste manufacturer’s specifications. The use of this overhead is to ensure that thicker PCBs and high mass components reach and exceed the solder paste’s melting point [2]. Applying peak temperatures of this magnitude disrupts the functionality of BGA/LGA style packages due to the component’s internal material composition. These higher temperatures often exceed the limitations of the component which cause it to unevenly expand and subsequently warp. As the PCB and/or component warp, the solder balls disengage from the pads. This leads to defective solder joints as either oxides form in between the pad and ball, or the pad and ball cool at different rates and fail to form a proper solder joint as seen in Figure 2. Both of these effects lead to the creation of HIP defects.