**PCBA Lecture Hall: Possible Causes of the BGA Pillow Effect (Head-in-Pillow)**

Head-in-Pillow (HIP) refers to the undesired phenomenon in solder joints, named for the resemblance of a person’s head resting on a pillow. The pillow effect (Head-in-Pillow, HIP) primarily describes issues associated with BGA components on circuit boards. Warpage or deformation from various sources can lead to a separation between the BGA solder balls and the solder paste printed on the PCB. As the circuit board moves through the high-temperature reflow zone, the temperature gradually decreases and cools. At this stage, the deformation of both the IC carrier board and the circuit board starts to revert to its original state (although sometimes it may not fully return). However, the temperature at this point is already below the melting point of both the solder balls and solder paste, meaning that both have solidified from their molten state. As the warpage of the BGA carrier board and circuit board gradually returns to its pre-deformation shape, the now-solid solder balls and solder paste make contact again, resulting in a false welding shape akin to a head resting on a pillow.

**HIP (Head-In-Pillow) Detection**

Based on the aforementioned theory, the majority of the pillow effect (HIP) tends to manifest at the edges of the BGA components, particularly at the corners, where warpage is most pronounced. To investigate this, one could utilize a microscope or an optical fiber viewer. However, this method typically only allows for observation of the outermost two rows of solder balls, making it challenging to assess those further inward. Additionally, when using this observational technique, it’s crucial to ensure there are no tall components nearby that could obstruct the line of sight. The current trend of high-density circuit board design presents significant challenges in this regard.


1. Additionally, the pillow effect (HIP) is often challenging to detect with current 2D X-Ray inspection machines, as most only inspect from top to bottom, leaving the position of any broken heads unseen. If detected, they can be rotated up and down, allowing for some observation of the X-Ray angle. Sometimes, it may be identified through in-circuit testing (ICT) and functional verification testing (FVT), as these machines typically utilize a needle bed method, which applies external pressure on the circuit board. This pressure can allow adjacent solder balls and solder paste to separate; however, many defective products may still enter the market. Such defects are usually quickly identified by customers who encounter functional issues, leading to returns. Therefore, preventing the pillow effect is a crucial concern for SMT.

2. Furthermore, you might consider burning the board (Burn/In) to filter out boards with HIP. By increasing the temperature during the burn, the board may deform, potentially revealing issues like empty or fake soldering. Thus, a self-diagnosis test must be incorporated during the burn process. If the location of the HIP does not align with the program’s circuit test, it may remain undetected.

3. Currently, the most reliable methods for analyzing HIP issues are Red Dye Penetration and Cross Section analysis, both of which are destructive tests and should only be employed when absolutely necessary.

4. Recently, advancements in [3D X-Ray CT] technology have made significant strides, effectively identifying shortcomings like HIP or non-wet-open (NWO) soldering, gaining popularity. However, the cost of these machines remains prohibitive.

5. Possible causes of HIP include factors originating during reflow soldering, with the actual issues often rooted in poor materials or practices at the circuit board assembly plant. This includes solder paste printing, placement accuracy of components, and the reflow furnace’s temperature settings.

6. Here are several potential causes of the pillow effect (HIP):

7. **BGA package**: Variations in solder ball sizes within the same BGA package can lead to the pillow effect, particularly if smaller solder balls are present. Insufficient temperature resistance of the BGA’s carrier board may also cause warping during reflow, contributing to HIP.

8. **Solder paste printing**: Inconsistencies in the amount of solder paste printed on the pads or the presence of via-in-pads can prevent proper contact between the solder paste and the solder balls, leading to HIP. Furthermore, if solder paste printing is misaligned, especially during assembly of multiple boards, there may not be enough solder to form a bridge, increasing the risk of a pillow effect.

9. **Precision of placement machine (Pick&Place)**: If the placement machine lacks precision or if the XY position and angle are incorrectly adjusted, misalignment between BGA solder balls and pads may occur. Additionally, when placing IC parts, if the Z-axis distance is not adequately depressed, some solder balls may fail to contact the solder paste, increasing the chance of HIP.

10. **Reflow profile**: Improper settings for reflow temperature or heating rate can lead to issues like insufficient tin melting or warping of the BGA carrier board, resulting in HIP. Rapid temperature increases in the preheating zone can prematurely volatilize the flux, causing solder oxidation and poor wetting. It is advisable to avoid excessively high or prolonged peak temperatures, adhering to the recommended temperature and time guidelines for components.

11. **Solder ball oxidation**: After BGA assembly in the PCB factory, probes are used for functional testing. If the probes are not adequately cleaned, contaminants may affect the solder balls, compromising soldering quality. Furthermore, if BGA packages are not stored properly in a controlled temperature and humidity environment, solder balls may oxidize, negatively impacting their bonding properties.