In the previous article, we talked about "How to Increase the bonding force for PCBA by using "Copper" base as the PCB surface finish“. Today, we will discuss further how the SMD (Solder Mask Defined) and NSMD (Non-Solder Mask Defined) pad designs of the BGA package affect the solderability and how these two types of pads affect the bonding force of PCA.
Increasing the solder contact area of a component terminal or leads will generally improve the solderability of the component, as it provides more surface area for the solder to wet and bond to the terminal. This can result in a stronger and more reliable solder joint.
However, it’s important to note that there are other factors that can also affect solderability, such as the type of surface finish on the component terminal and the solderability of the solder itself. So while increasing the solder contact area can be beneficial, it may not be the only factor to consider when trying to improve the solderability of a component.
II. Increasing the solder contact area of the component’s terminals or leads
Please note: what I mentioned here is to increase the contact area of the component’s solder terminals or leads, not to increase the amount volume of solder.
Before reading this article, it is recommended that you refer to the article of "Concept Clarification of Electronic Component Soldering Strength" by workingbear first.
Workingbear must emphasize again: "The soldering strength is basically proportional to the soldering contact area". Without considering the replacement of the solder paste formula and the surface finishes of the PCB (Printed Circuit Board), there are two key points to enhance the soldering strength of the component:
1. Increasing the solder contact area of the component’s terminals or leads. It’s not the amount volume!
If only more solder is added to the solder balls of the BGA, its solder strength will not increase basically, because its solder pad contact area remains the same. For SMD components with leads (such as QFP) or side solder terminals (such as the capacitor), increasing the amount volume of solder may increase its solder strength, because the leads of these SMD components have side terminals that may have solder climb up to higher position and increase the solder contact area.
If the solder amount can be increased to allow the solder to form a complete curve on the side of the solder foot, the solder strength can be effectively increased. Just like adding a support on the side of a tree or a utility pole to prevent it from falling down in the event of a typhoon, adding an R angle at the corner of a mechanism design is also the same principle, and the arc-shaped solder can effectively disperse stress.
Therefore, if increasing the amount of solder can allow the solder to climb up the sides of the solder terminals or leads, and even make the side solder climb higher to completely cover the solder leads, then its strength will increase significantly. This is much better than just welding at the bottom of the solder leads because you have increased the soldering area and its soldering structure has been strengthened.
Unfortunately, the side walls of most SMD component leads are not electroplated directly on bare copper due to cost considerations, which causes them to easily oxidize and cannot be soldered. Otherwise, they are electroplated, but the pitch between the lead is too small, causing the design of the PCB solder pad to be afraid of short-circuits and unable to extend so that the solder cannot completely climb up its side to form a perfect arc.
Since climbing solder on the side walls of the solder lead can increase the solder contact area and strength, many people focus on the grounding foot (GND), such as increasing the circular or semi-circular holes on the ground pin of the B2B connector at both ends or designing it into a "U" shape, which all have the opportunity to increase its solder strength.
Since we’re talking about how increasing the soldering contact area can help to enhance the strength of the solder, we have to briefly mention the pros and cons of SMD and NSMD pad designs for BGA packaging on the PCB side.
Please note that we should not confuse the "SMD (Solder Mask Defined) pad" with the "SMD (Surface Mount Device) component" here.
Assuming that the exposed area of SMD and NSMD pad designs is the same, the solderability of NSMD pads should be better than that of SMD pads. As mentioned earlier, this is because NSMD pads will wet the side walls of the pads during soldering, while SMD pads do not have side walls due to being covered by the solder mask. (NSMD is also called "Copper Defined pad.")
But don’t rush to change all BGA pads to NSMD designs immediately because there is no perfect solution if BGA cracking! In this fair world, even if the exposed pad surface of SMD and NSMD seems to have the same size, the actual size of the SMD pad is much larger than that of NSMD (depending on the layout design). This is because a large part of the SMD pad is covered by the green solder mask actually, and it is easy to be deceived if you don’t look carefully. Because the size of NSMD pads is relatively small, almost only slightly larger than the solder balls of BGA, the ability of SMD pads to withstand pulling force will be relatively poor. Once the BGA solder ball breaks, it is often seen that the NSMD pads are pulled up together with the solder balls.
As mentioned by the workingbear before, stress will find the weakest point to release. When the pad changes from NSMD to SMD and the soldering strength increases, the ability of the solder IMC layer to resist stress becomes greater than the bonding force of the PCB copper foil attached to the substrate (because the size of the SMD pad becomes smaller), and the breaking point shifts to between the pad and the FR4 substrate. Therefore, the workingbear still believes that if you want to completely solve the problem of BGA cracking, you should try to reduce stress, which is the only way to achieve the best improvement.
Therefore, the conclusion is that NSMD pads have better solderability than SMD pads, while the bonding strength of SMD pads is better than that of NSMD pads.
2.Using Through-Hole component to Replace Surface Mount device
In fact, no matter how much the solder strength of electronic components using surface mount technology (SMT) is improved, its ability to resist stress is limited. To further increase the strength of the solder, the stress needs to be transmitted to other structures through mechanism design. The most effective way to achieve this is to design the pins as upright plated through-holes (PTHs), so that the stress on the pins can be transferred to the hole walls of the printed circuit board (PCB) for support, which in turn can increase the strength of the solder. The common practice is to change some of the surface-mount soldering of the component pins to through-hole soldering, such as the iron frame soldering pins of Micro-USB connectors. They still go through the SMT process, but some of the pins are produced using the paste-in-hole (PIH) process. The latest Type-C connectors also have parts with a mix of through-hole and surface-mount soldering.
In addition, for ball grid array (BGA) packaged components, we can consider to layout vias on the pads, just like a rivet to fix the pad on the FR4 material. This is similar to the idea of anchoring a house with ground screws to prevent seismic damage. However, the vias on the solder pads must be electroplated and filled, otherwise, the BGA’s solder balls may form voids or bubbles, which can lead to severe defects, such as the head-in-pillow effect.
Recommended reading: Principles of via-in-pad handling.
Article series :
- Why BGA soldering ball always crack(1)? Stress > bonding-force
- Why BGA soldering ball always crack(2)? The composition of PCBA bonding-force
- Why BGA soldering ball always crack(3)? IMC layer growth is a certain result to form the soldering joints
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