Solder voids are the empty spaces or unplated areas in a PCB. They can happen due to a wide range of reasons, some of which are solder paste oxidation and when the low heat temperature is used.
In this article, we help you understand some of the possible options to explore to get rid of solder voids in a PCB.
How are Solder Voids Formed?
Solder voids typically refer to the “void” or absence of solder. It commonly happens when there is an empty space created on the PCB, due to the non-application of solder in that area.
Also called solder joint void, it can be detrimental to the circuit’s performance, due to the general instability and general weakness created in the PCB in the process.
How to Fix Solder Voids in a PCB
Once you notice the empty space, the next step to take is fixing it. Here are some suggestions on how to fill the voids in solder joints of a PCB.
1. Reduce the Solder Paste Quantity
Sometimes, the quantity of solder paste applied is responsible for the voids created in the solder joints. The logical thing to do here is to reduce the amount of solder paste deposited.
Here are some additional considerations:
- Go for Lower Volatile Content: it is better to go for the solder paste that has a lower volatile content. The reason is that it doesn’t absorb much moisture, which, in turn, reduces the chances of creating void solder.
- Be Mindful of the Quantity: despite the fact that a low-volatile solder paste content works best, it doesn’t need to be much. So, apply a little quantity.
2. Extend the Preheat Time
The time spent in preheating the solder is very important. Here are some of the reasons why it matters:
- The preheat time’s extension aids the possibility of having the solvent reaching the optimum temperature.
- Once the optimum temperature is reached, the solvent in the flux will make a complete vaporization.
- Through this process, it is easier for the residue of flux left behind in the solder joints to be lowered.
3. Modify the PCB’s Stencil
The stencil of the Printed Circuit Board (PCB) can do with some modifications. This consideration is based on the fact that some PCBs are prone to solder voiding than the others are.
To this end, here are some ideas on how best to modify the board’s stencil:
- Go for the “Gas Escape Routes”: it is better to go for or use the PCB stencils that have, give or provide gas escape routes under the components.
- The idea is to create clearances around the holes. That way, the solder paste will not be pulled into the PCB during the reflow process.
4. Modify the Reflow Profile
This refers to the process of extending the soak time. The modification of the reflow profile provides these advantages:
- It helps to generate less-volatile material.
- The reflow profile’s modification is also a good way to create more time for the gas to escape.
Here are some ideas on how an increase in the soak time can reduce BGA void formation in PCBs:
- A soak time extension is tenable in the driving off of more of the low-boiling volatiles.
- These volatiles evaporate and escape from the solder paste before the reflow (solidification).
What is the Content of Solder Void?
The content of solder void includes air and flux, which have been trapped within the solder joint. It is also possible to have a cured resin content, especially when a solder paste containing resin was used for the soldering process.
Causes of Solder Voiding
The following are some of the notable causes of solder voiding in a PCB:
Limited Outgassing Paths
Outgassing paths are the channels or pathways from where gasses can escape from the circuit board. The absence of or a limited number of these outgassing paths tend to create solder void, as the gasses that ought to have escaped are then trapped inside the board.
Paste-Induced Oxidation
Solder void is also likely to occur when the solder paste oxidizes.
Low Preheat Temperature
There is a possibility of solder void formation when the preheat temperature is lower. The reduction also means that the solvent in the flux will have difficulties without making a complete vaporization.
The Design of the PCB Stencil
The type of stencil the circuit board uses could be a contributing factor to the formation of solder voids.
As a rule of thumb, you should consider changing the current stencil if it is prone to solder voids.
Excessive Flux Application
When excessive amounts of flux are applied to the solder paste, the formation of solder voids is likely.
To this end, it is better to apply a little flux. This way, there wouldn’t be an issue with the flux making an “outgassing” before the solder is solidified or enters a “solid-state.”
The Solder Paste’s Quality Matters
What is the quality of the solder paste like? As a rule of thumb, the solder paste tends to shrink. An example is the lead-free solder paste, which shrinks up to 4% when cooled down to a solid-state.
The idea is that solder void becomes likely when large pads are evenly cooled down. This is in addition to the possibility of spacing and shaping in the solder.
Short Soak Zone Time
Solder voids also occur when the soak zone time is short, especially during the reflow process.
What Types of Solder Voids Exist?
Different kinds of solder voids exist and they work differently. Here is a summary of them:
Pinhole Microvoids
This is a type of solder void that is located between the copper land and the IMC layer. It usually measures between 1 and 3 microns in diameter.
Macro Voids
As the name suggests, this is one of the larger types of solder voids. Measuring up to 12 mils in diameter, macro voids are formed when there are trapped gasses and solder paste.
Intermetallic Microvoids
These are the sub-micron sized solder voids that are created due to the exposure of the PCB to elevated temperatures. The voids are created when the board’s exposure to over 100-degree Celsius of temperature happens for over 48 hours.
Planar Microvoids
These are the BGA voids responsible for failures in the Ball Grid Arrays (BGAs). Planar microvoids measure between 1 and 2 mils in diameter.
Microvia Voids
These are caused by the microvias associated with the Ball Grid Array (BGA) joint.
Shrinkage Voids
In addition to being susceptible to shrinking, these solder voids are peculiar to both the Through-Hole (THT) and Ball Grid Array (BGA) solder joints.
The easiest ways to identify these voids are:
- Taking note of the elongated and rough voids that run from the surface of the solder joints.
- Noticing the cracks in the solder joints.
Solder Voids vs. Plating Voids
Voids occur in the Printed Circuit Board (PCB) when certain parts or spaces are left unplated. Both the plating voids and solder voids are the two major types of voids in circuit board design.
Here are some of the differences between the two:
- Plating voids happen when the electroless copper plating process is being used. In this case, the copper plating doesn’t make a complete covering of the Through-Hole’s inner walls. On the other hand, solder voids occurs when sufficient solder paste is not used.
- It is almost easier to resolve solder voids than the plating voids and this is due to the simplified processes, such as switching to a solder paste formulation with lower void-forming tendency.
How Do Solder Voids Affect PCBs?
These are some of the ways that the formation of solder voids in a PCB could be detrimental to the board’s performance:
- The reliability of the solder joint wouldn’t be fully assured.
- The current-carrying capacity of the PCB may be affected.
- The formation of solder voids also causes solder bridging in the PCB.
- The formation of solder voids also causes the irregularity of the solder flow. For example, it could cause the solder to flow from one pad to another, especially during the solder reflow process.
- The thermal-conducting capabilities of the solder joints are also reduced in the process.
How to Prevent Solder Voids with the Industry Standards
Following the industry specifications as they pertain to the control of solder voids in PCBs can help. The IPC had set-out these standards in the IPC-A-610, which spelt-out the following criteria for solder voids:
- It specified that the solder voids are to be considered “defects” if the overall calculated area of voids in any solder joint or solder ball is more than 25% of the total solder ball’s area.
- The defect is to be monitored using an x-ray image.
Conclusion: Are Solder Voids Problematic for PCBs?
No circuit board is “perfect.” One issue or the other could cause it to be defective and the solder voids are not an exception. However, the voids are not much of a problem, as they could some definite purposes, such as providing stress relief for the solder joints and limiting the chances of cracks within these joints.
Solder voids become a PCB problem when they occur in certain locations. For example, the occurrence of these voids on the circuit board’s interfacial surfaces could trigger cracking under certain atmospheric stress conditions.
We will help you evaluate the best ways to optimize your PCB so it doesn’t become overly exposed to solder voids.
