A solder ball is a glob of solder that can serve to connect a chip package and a PCB. You can also use solder boards to connect stacked packages in multichip boards.
They can be mounted on circuitry boards manually or through automated equipment. Their placement is usually secured through a tacky flux.
However, solder balls are a double-edged sword. Solder balls are also the most common defect that detracts from SMT assembly processes. A solder ball placed within 0.13mm of traces, or that is wider than 0.13mm in diameter violates the principle of minimum electrical clearance.
The errors that can cause a solder ball to bring about defects in an assembled PCB are innumerable. According to IPC, solder balls do not cause defects so long as they’re held firmly in place. This article examines the good, the bad and the ugly of solder balls.
1、What is a Solder Ball?
Solder balls are also called a solder bump or solder sphere, due to their geometry. A solder ball is a spherical piece of soldering used to connect chip packages to PCB.
Solder balls are created through sequential flow/quench or reflow processes. After passing through these processes, they’re then degreased and classified.
You can increase a solder ball’s contact reliability by flattening its ball shape into a coin shape. We call such a solder ball a coin solder ball.
When Solder Balls are Bad News for PCB
A solder ball can also cause defects in a PCB. They can undermine the electrical reliability of a PCB electronic. Solder balls within 0.13mm of traces, or with a diameter wider than 0.13mm violate the principle of minimum electrical clearance.
Also, the IPC A 610 standard stipulates that even solder boards that are <=0.13mm in diameter can cause defects. Such defects occur when five solder balls with the stipulated diameter are placed with 100mm^2.
You can inadvertently create detrimental solder balls during automated reflow, as well as during hand soldering. When the solder ball is not warped in the no-clean residue or the conformal coating, the solder ball becomes a bane.
However, it could be complicated to determine whether the ball is entrapped in a no-clean residue or conformal coating. However, one natural and reliable way that you can use to determine that is by brush sweeping it.
If it remains in place after you stroke them with a brush, then it won't cause any defects. That's the IPC's take on the matter.
There are some other ways to troubleshoot a problematic solder ball. The most effective troubleshooting method is one that first tries to identify the stage where the unintended solder ball occurs.
The defective solder ball can occur either during the printing process, the pick-and-place process, or the reflow process.
Solder balls are integral parts of most consumer electronics. However, as consumers are increasingly demanding smarter, powerful and portable electronics, it's become more important to get it right with solder boards.
However, solder balls remain one of the most complex and delicate components of electric circuitry. Its use requires a high degree of conscientiousness.
Want to learn about the best way to deploy solder balls? Read on.
2、Solder Ball Valve
A solder ball valve can act as a connector between multiple chip stacks in a PCB. It can serve to regulate the flow of electrons and signals between the various layers of the stack.
Solder ball valves are usually contained in a Ball Grid Array (BGA). A BGA usually provides more inter-connectivity than a dual in-line or flat package.
How to Solder a Ball Valve
The method described here revolves around of placing solder ball valves on BGA packages through the use of a ball pickup tool. The primary objective is to form an array of solder balls on a substrate.
You’ll use this substrate to interconnect the conductive sites on other substrates.
To implement this method, you'll need a ball pick up tool. This ball pickup tool uses vacuum suction to pick up solder balls from a fluidized ball reservoir. That implies that you need a solder ball reservoir that contains a collection of premade solder balls.
The reservoir should also come with attachment agents. That also implies that you need at least one vacuum. That also implies that you need at least one vacuum source to provide the orifice of the tool with suction power.
The vacuum suction tool has at least one orifice for picking up a premade solder tool. It also comes with a ball seat that's connected to a vacuum source and a pressure source, controllable.
The tool deploys a jet of gas to inject the picked solder balls into the conductive sites of a substrate.
In another arrangement of the technology, the pads of the substrate are placed in a fluidized ball reservoir. The coating is applied with a flux or adhesive that attracts and bonds with the solder balls in the reservoir.
Still, want to learn more about the best ways to use solder balls? You'll find that out in the next chapter.
3、How to Make a Solder Ball?
One of the oldest and widely used solder ball creation methods is the 3-Orifice design. In this method, you start by first acquiring a solid solder alloy, preferably a Sn63Pb37 or a Lead-free solder.
Craft the solder alloy into a solder wire or a solder sheet. For a wire, cut the wire into tiny pieces, and for a solder sheet, knock out specks. Cut out pieces and specks in measures that will accurately yield the volume of a solder ball with a 2mm diameter.
Next, place the pieces and specks into a column of hot oil to melt. The upper section of the column of hot oil should have a temperature above the melting point. Moreover, the lower section's temperature should be below the melting point.
You’ll obtain your desired solder balls when the pieces and specks in the column of hot oil melt. Next, cool the balls in a viscous liquid.
Note that the presence of oxides in the column can distort the spherical shape of the balls. However, you can place a film of flux over the column to prevent this.
This method is highly efficient and low-cost. With this method, you can create up to 7,000 high-quality solder balls per second in any orifice. However, the method also comes with its downsides.
For starters, the technique can be fraught with contamination and turn out messy. Also, each of the balls will each have a different weight, although you can measure their weights. Also, it’s nearly impossible to obtain balls with a tolerance of 1.5%.
The Importance of The Packaging on The Formed Solder Ball
As noted above, the presence of oxides can distort the shape of the solder ball. One way to prevent the oxidation of the solder balls is through packaging.
Not only can packaging prevent the removal of oxygen, but it can also prolong the shelf-life of the solder balls even beyond their expiry date.
Want to learn more about how to avoid defective solder boards? The next chapter holds the answers.
4、What Causes Solder Balls During Hand Soldering?
Here are some of the cause of solder balls.
The presence of moisture in your solder paste can cause solder balls to explode during reflow. Moisture seeps into the solder paste during refrigeration.
If the paste does not attain room temperature after you remove it from the refrigerator, it will soak up the moisture. However, you can eliminate the moisture by baking.
The board itself can bring about undesired solder balls during hand soldering. The presence of air, moisture or alcohol used to clean the board can make the board yield unwanted solder balls.
These contaminants can creep in between layers, open vias and through-holes of a board with cracked or incomplete plating. Moreover, when the board undergoes heating in reflow, these contaminants are forced out. Their abrupt escape shoots gases in all directions and consequentially blows liquid solder across the board. Paste flux gases may also produce this effect when they escape from the sections of the board close to the surface.
If your board is admitting contaminants through open edges and vias cracks, and through-holes, there isn’t much hope. You’ll most likely need to remake the board, as you can’t bake the trapped air away.
If paste gasses are jetting off from underneath the board, you can resolve this by reducing the amount of paste. You can also resolve this by minimizing the amount of volatiles in the paste.
Your stencil might be depositing solder paste discriminately. You need to ensure that the under stencil cleaning process you use is efficient and thorough-going.
It could be that you're using an inappropriate under stencil wiping roll which is too thick. The inappropriate thickness of the roll can cause the balls to spread across the underside of the stencil. Moreover, when you eventually use the stencil on a PCB, the extra balls get deposited on the board.
Inappropriate Solder Paste Formulation
Poorly formulated solder pastes may explode during heat reflow, and blow liquid solders randomly across the board. The volatile materials are usually the most likely causes of the explosion.
In such cases, you can prevent such explosions by reducing the pre-heat ramp rate. That allows the volatile material to pushed out without abrupt outgassing. However, you need to ensure that you preheat slow enough.
The Best Troubleshooting Technique For Solder Balls that Occur During Hand Soldering
The best way to find out the causes of solder balls during hand soldering is to test multiple products. The aim is to check to see if the defect occurs in certain types of PCB. Run multiple boards with the same solder paste and equipment to pinpoint the exact variables from which the fault originated.
In the next chapter, we will delve into the reliability of solder balls and what troubleshooting techniques to use.
5、The Solder Ball Joint Reliability
Studies have been conducted to show the effects of Pd film thickness on the solder ball joint reliability. The subject of the study was electroless Ni/Pd/Au plating on an Sn-3.0Ag-0.5Cu (SAC305) solder ball joint. The study used a solder ball shear test.
The Pd film thickness between 0.05-0.02 microns was found to be the optimum for solder joint reliability after multiple reflow cycles. Studies have also shown that solder ball joints are more reliable when using 0.02 micron thick electrodes.
This result is even better than that obtained using electroless Ni/ Au plating.
The study also shows that the shape and thickness of the intermetallic compounds (IMCs) determine a solder ball’s reliability. In particular, the degree of adhesion at the dendrite layer of the IMCs/solder interface immensely influenced a solder ball’s reliability.
It was also shown that (Cu, Ni, Pd)6Sn5 IMCs that contained minute amounts of Pd yields excellent solder ball joint reliability. Mainly because of Pd inhibited the growth of IMC.
Want to explore further the issues that limit the reliability of the solder balls? Read on
6、Problems and Defects
The IPC A 610 standard stipulates that five solder balls with diameters <=0.13mm are not supposed to be placed within 100mm^2. However, this isn’t the only reason for a defective solder ball.
The solder might run on a wet track because of a poor resist coating. An unreliable coating may fail to adhere to a track's tin/lead coating. The coating can also fail as a result of poor print thickness control. You need to be very careful when removing solder balls resulting from moisture-induced poor resist coating. You could easily damage the track in doing so.
Random solder balls can occur due to the spitting from the wave. This defect is therefore linked directly to the wave soldering parameters. The separation of the wave can be attended by the placement of the solder in the distance away from the tracks.
In such an instance, the solder may bounce back off the bath.
Also, a solder ball may result if you set the preheat incorrectly, or you increase the flux quantity inappropriately. In this case, the solvent will escape from the flux defectively.
You can identify this problem by hovering a glass plate above the wave. You should be able to see some bubbles at the bottom of the glass as the glass contacts the wave. The fewer bubbles you spot, the better.
You also need to confirm that the resist and flux are compatible.
Exploding Volatile Materials
The occurrence of random solder joints can also be caused by explosions caused by the volatiles residues in the flux. You can resolve this by placing a piece of the white card above the wave, leaving it there while the wave runs.
It would help if you did not process the board while doing this the first time. Then afterward, run the board through the machine while the piece of the white card remains in place. You’ll most likely identify the culprit from there.
Many reasons can cause defective solder boards. Below is a rundown of the most common causes:
The lack of solder masks between adjacent pads.
The preheat temperature isn’t high enough to activate the flux.
The lack of adequate space between adjacent pads.
Improper placement of elements on a circuit board.
Solder residues left behind on PCB surfaces and pads.
Solder paste squeezed out due to extremely high pressure from placement.
The use of pastes in excesses and the occurrence of paste slump.
An unclean stencil smeared with solder paste on its underside.
Misalignment of solder paste during printing.
The Best Troubleshooting Methods
1. Make sure the mat and template are compatible, and the dimensions are right.
2. Cleaning the stencils as quickly and as thoroughly as possible.
3. Adjust the solder paste printing pressure.
4. Eliminating the gap between the PCB and the stencil.
5.Using additional solder mask between the pads.
6. Adjust the pressure to pick and place the nozzle.
7. Separating the new flex from the old flex.
One way to ensure you get things right is for you to use high-grade observation tools. Check up on any place a solder paste is used during every process using a microscope or x-ray.
Regardless of the type installed components and the PCB, or whether to clean the panel, you can use these tools to observe. However, doing so, and also using the information in this guide, you'll make the best use of solder boards.
Nonetheless, you should be able to get assistance from your manufacturer. However, you need also to ensure that the technical assistance you're getting from your manufacturer comes from a seasoned technician. Not all the representatives from your manufacturer are knowledgeable enough concerning the peculiarities of your PCB electronics.
However, we have a solid track record and a full breadth of experience with solder balls and PCBs. You can tap into our deep knowledge reservoir of SMT processes today.