Working for manufacturer engineering couple years. Sharing the manufacturing experience, skill, DFx, design, and information. Be care of this blog is personal share and content may not be 100% correctly.
BGA packages are prone to HIP, also known as HoP defects, during the reflow process, and it is an undeniable fact that the larger the size and the more balls a BGA package has, the more likely HIP is to occur. This double-ball soldering defect, Head in Pillow , is a particularly troublesome engineering problem because neither 2D X-ray inspection nor electrical testing (FVT or ICT), and even sending the product to burn-in (B/I), can detect HIP issues with 100% certainty.
As a result, defective BGA ball solder joints may occasionally be returned from the market after the product is delivered to the end customer.
So, is there a way to solve or reduce the occurrence of HIP (Head-In-Pillow) or HoP (Head-on-Pillow) soldering defects in BGA soldering?
The answer is not a simple yes, nor can we guarantee a 100% solution to HIP/HoP problems, because HIP issues are related to the IC substrate, assembly PCB, reflow oven temperature, product design, internal and external stresses, and other factors. It is difficult to solve everything at once.
Below, Workingbear can only try to provide some current solutions and directions for HIP/HoP defects for your reference.
1. Slowing Down Reflow Heating to Reduce Board Warping Risk
Whether it’s an IC substrate or an PCB, if the reflow oven heats up too quickly, it can cause uneven temperature distribution on the substrate or PCB, leading to localized thermal stress. Temperature is a major factor causing deformation in IC substrates and PCBs. To thoroughly solve this problem, we might need to look at it from a thermodynamic perspective. For example, adjusting the orientation of the glass fibers in the board to resist greater deformation or ensuring a more even distribution of copper foil on the substrate or PCB to minimize the impact of thermal stress.
In the SMT process, the biggest issue with uneven temperature distribution is often due to a rapid heating rate. This can cause localized areas to lag in temperature rise compared to the surrounding environment. Examples include large copper areas on the PCB, which need to absorb more heat before their temperature increases, and large plastic components, which absorb heat more slowly and can block heat transfer to their solder joints, such as smart card readers. Additionally, components located under shielding cans also struggle to absorb heat transferred through hot air.
To address this, try slowing down the reflow heating rate by reducing the conveyor speed or lowering the temperature in certain heating zones. This can help achieve a more uniform temperature across the PCB. However, be cautious not to let the PCB and components absorb too much heat, as this could cause other issues like brittleness or bubbling. It’s recommended to conduct experimental plans to obtain the optimal temperature profile.
Additionally, consider using “thermal relief pads” design to improve the overall temperature uniformity of the PCB.
2. Pre-baking BGA Packaged ICs or PCBs
If BGA packaged ICs or PCBs absorb moisture, they are prone to delamination and warping when passing through the high temperature reflow oven. Pre-baking at 105°C~120°C can help remove moisture and reduce deformation during reflow.
Since warpage in the BGA IC substrate and PCB can cause the solder balls and solder paste not to contact properly, leading to poor solder joints, increasing the solder paste volume, more accurately, the thickness of the printed solder paste, can help. This should shorten the distance and increase the contact area between the solder paste and balls, improving yield.
However, increasing solder paste volume is a complex issue. There is a risk of shorting adjacent pads if too much paste is applied. Instead of increasing solder paste on all solder balls, it’s more effective to target areas with greater deformation.
Here’s a selective method: statistics show that 99% of HIP issues occur on the outermost rows of BGA balls, particularly at the corners. Therefore, increasing the solder volume on the outermost BGA pads should suffice. This can be done by making the stencil openings square for the outer balls while keeping the inner openings round.
Above picture shows the actual stencil aperture result for BGA package with ball sizes less than 0.4mm in diameter.
Sometimes, due to design constraints, thinner PCBs (e.g., 1.2mm, 1.0mm, 0.8mm) must be used. Thinner PCBs are more prone to warping in the reflow oven. Other factors contributing to PCB warping include:
Larger size of PCBs are more likely to warp due to gravity.
PCBs with heavier components on it.
PCBs undergoing multiple reflow cycles (each reflow cycle takes the PCB above its Tg temperature, potentially causing irreversible changes).
If the previous three methods don’t work, using a reflow carrier might be the last resort. Reflow carriers, with their support pins and registration holes, can effectively prevent PCB warping during reflow. While reflow carriers can be costly and labor-intensive, the cost is negligible compared to the losses from HIP defects.
Tg, or glass transition temperature, is the temperature at which a material transitions from a glassy to a rubbery state. PCBs with a low Tg value soften quickly in the reflow oven and remain soft for longer, leading to more severe warping. Using high-Tg materials increases the PCB’s ability to withstand stress and deformation, though these materials are more expensive.
6. WBGA IC Packages Are Particularly Prone to Warping
WBGA IC packages seem to be more prone to warping due to their inherent structure. The image above shows how these WBGA IC packages are more susceptible to deformation issues. IC manufacturers must strictly control the deformation of their ICs.
If the previous three methods don’t work, using a reflow carrier might be the last resort. Reflow carriers, with their support pins and registration holes, can effectively prevent PCB warping during reflow. While reflow carriers can be costly and labor-intensive, the cost is negligible compared to the losses from HIP defects.
Leave a Reply