The Worst – and Best – BGA Assembly. Ever.
So, you went through the whole rework/assembly process and the BGA looks amazing sitting on the board. Unfortunately, looks can be deceiving in this business. BGAs hide their problems very well, so you do need an x-ray machine to look through the package to assess the quality of the assembly. The x-ray image in this x-ray image is the perfect example of the best – and at the same time worst – BGA assembly we’ve ever seen. It’s the worst because it has every single assembly problem you can find in a BGA assembly. It’s also the best because it is a great educational tool, since it allows us to show you all these problems in a single image. This image was taken with a TruView™ Fusion x-ray inspection system at 110kV and 100uA, with the sensor tilted at 55°.
In the following sections, we’ll show you what you don’t want to see in the x-ray image of your BGA assembly.
Short circuits, or “shorts”, are identified by a solder bridge between two or more solder balls. This defect is easily identified in an x-ray image. The root cause of this problem is diverse, including:
Incorrect solder deposition. Either too much solder was deposited or it was placed in the wrong place in between the pads.
Misalignment between BGA and board. There’s some misalignment the BGA can accommodate and correct by the superficial tension of the solder balls. However, excessive misalignment can cause the solder balls to short.
Defective solder mask. If the solder mask is not properly deposited onto the board, some of the solder on the ball can run on the trace, eventually touching the neighboring ball and causing the short.
You have an open circuit, or “open” in your BGA assembly, when the solder ball fails to collapse and doesn’t touch the pad on the board. Unlike its close cousin, the head in pillow, an open is always an open and can sometimes be discovered during electrical tests.
Opens are either caused by insufficient solder on the pads or by BGA to board misalignment or coplanarity problems. It is also possible that the solder on the pad ran to the trace, thus leaving the pad without enough solder height to allow the collapse of the solder ball. You need an x-ray machine with enough magnification and resolution to properly identify the separation between the solder ball and the board pad. Your x-ray machine also needs tilting capabilities, also known as 2.5D x-ray inspection.
HEAD IN PILLOW
Perhaps the most challenging BGA assembly problem to identify, head in pillow (HIP) has haunted engineers since the early days of the BGA history. Under special circumstances, the solder ball collapses – but not entirely. This partial collapse allows the solder ball to rest on top of the pad (like a head in pillow) and to make electrical contact. However, since the contact between solder ball and board pad is mechanical and not metallurgical, the integrity of this electrical contact will depend on how the board is flexed. That’s the case you’ve experienced where the finished board passes all electrical tests in your facility, but it fails at the customer’s site. Temperature differences that cause the BGA to move, even slightly, can cause the solder ball to open.
One of the most popular questions we get asked in a daily basis is: How to find HIP? Although we keep improving our algorithms to allow you to automatically and unequivocally find this defect, it is still a manual process in the industry. In order to attain an adequate image you need an x-ray inspection machine with high magnification, resolution and geometric maneuverability. A wide range of geometric maneuverability will allow you to capture profile acquisitions (side view) as well as a tilted view of the BGA assembly. Some call it 2.5D inspection.
The percentage of voiding inside each BGA solder ball is the key performance indicator in the BGA assembly process. The IPC-A-610D Standard1 (Acceptability of Electronic Assemblies) categorizes >25% BGA joint voiding as a defect. Although some experts debate the validity of this number, it still is widely accepted as a reasonable threshold to define pass or failure of a BGA assembly.
Voids are caused when the entrapped flux from the solder paste volatilizes and rises in the molted solder paste. Due to buoyancy voids are usually located near the package interface.
Several articles have been written about solder voids and how this problem can be mitigated, so we’re not going to repeat them here. Suffice to say that this problem has both a process and a chemistry solution, and both solutions must be investigated. Here is an example of a process versus a chemistry solution: one of our customers recently solved a serious voiding problem by applying a longer dwell time below liquidus, instead of jumping to a very expensive solder paste.
To measure the void inside each solder ball you need to first define where the voids are. Once that’s done, you can calculate the ratio between total ball area and the void area. This percentage will determine if the assembly passes or fails based on a specific criterion. If you’re following IPC-A-610, this criterion may be 25% of void. To make your life easier, all these calculations can be done in software. The TruView™ software automatically identifies the solder balls in the image and measures, in a per ball basis, the total void, the largest single void, the eccentricity (how round the shape is), the diameter, and the area. The TruView™ software also includes tools that go beyond pass/fail criteria and allows you to visualize individual BGA balls using 3D rendering.
Imagine two planes, like two sheets of paper, perfectly flat. Now imagine you place these two sheets of paper on top of each other. The four corners of the top sheet will touch the four corners of the bottom sheet. That means they are coplanar – they’re perfectly parallel to each other. Now gently lift one of the corners of the top sheet. You have just created a coplanarity problem, a common issue in BGA assembly caused by either assembly misalignments or by popcorning due to trapped moisture in the plastic BGA package.
Coplanarity is one of the trickiest problems to diagnose because it requires the inspection of the whole BGA. A single area of the component will not tell you the whole story. However, as we analyze the x-ray images of all components, the story begins to clarify.
The ideal shape of a solder ball is round. Thus, when looking at the x-ray of a BGA assembly you hope to see a grid of dark circles. Unfortunately, that’s not always the case. In the images you can see an example where the solder ball is oval. The main reason behind this problem is alignment between the BGA and board. If the solder ball is not right on top of the pad on the board, it will elongate (due to superficial tension) to make the metallurgical connection, thus creating this oval shaped profile. Another issue we’ve seen is the excessive deposition of solder on the pad. This excess solder doesn’t really have anywhere to go, so it balls up in different shapes and forms.