BGA (Ball Grid Array) appears as an evolution of PGA (Pin Prig Array). It is an Surface Mount Technology SMT (Link it to SMT articles). In the race of downsizing chips the need of high-density package technology increased, so pins become pads. These pads need to be soldered by solder balls. We’ll go through the advantages in BGA technology, the soldering process and some difficult that appears on it.
|Instead of leads BGA uses solder balls. This provides higher SMT assembly reliability and allows to reach smaller balls pitch which increases the density of miniaturization. The balls pitch, distance from the center of one ball to the center of the next defines what type of BGA technology we are using. One millimeter pitch is standard BGA if we go smaller than that we’re talking about micro-BGA. Micro-BGAs has pitches of 0.6, 0.4 and even 0.3 mm.|
Each BGA would be identified by the number of sockets that contain, for example BGA 370 means 370 sockets. The BGA package contains a PCB on where the silicon die is placed, this is a high quality PCB like the one used for motherboards. Commonly uses fiber-reinforced material as BT substrate (Bismaleimide Triazine). When more flexibility is required polyimide tape is also available. Conductors are traces etched in copper foil bonded to a polymer substrate. Through-hole plated vias use allows several layer of interconnection.
BGAs are available in plastic or ceramic bodies, another option is metal-core BGA. Lower cost of plastic bodies make them more commonly used. Ceramic packages are vastly used for telecommunications, device-under-test equipment applications and laptops. Metal-core allows to use more circuitry than other options mentioned, mini-circuitry can be placed inside the BGA package, this an a addition to the regular number of balls and circuitry already there.
BGA Technology Advantages
We have strong reasons for choosing this technology, most of them are mentioned in the list below:
- Higher pin density:We can now have hundreds of pins on a single package without compromising quality of the soldering neither package reliability.
- Lower inductance leads:unwanted inductance is directly proportional to distance, so less lead length provide us less unwanted inductance.
- Better heat conduction:Less leads distances ensures less thermal resistance also providing as result better flow and conduction heat in between the two components that allows better conduction heat through the board.
- Increased performance:As a result of all advantages mentioned before combined. Better electrical performance compared to other IC packaging technologies. Also provides superior performance at high speed.
BGA Package Disadvantages
- Noncompliant connections:Since connection is made of solder balls instead of leads, this elements don’t have flex capability therefore they are not mechanically compliant. Mechanical or thermal stress can fracture solder joints. Anyway, different techniques has already been applied to diminish this disadvantage. Just for naming one for example a compliant layer is added in the package that allows the balls to physically move in relation to the package.
- Difficult inspection:Potential faults became difficult to identify and fix, since solder joint is not at the surface like in other assembly technologies. X-ray is needed for this type of inspection, this increases control time and costs.
- Harder for prototyping and development instance:Imagine that using this type of solder for BGAs development is not practical, so sockets are used instead. Socket are unreliable
- More expensive:The bumping process, the substrate and inspection costs become higher costs compared to a QFN package.
BGA Component Soldering Technologies
A simple explanation of the BGA soldering process would be:
1. Solder paste is printed on pad array on PCB, this could be stencil or flux is coated onto pad.
2. Pick and place automated machine places BGA components onto PCB, here the alignment is critical.
3. PCB is ready to go reflow soldering in reflow soldering oven.
Key factors to consider in BGA soldering process
BGA Components storage
BGA are a thermal-sensitive and humidity components. The storage environment should be dry and temperature controlled. Typically uses temperatures from 20°C to 25°C and less than 10%RH humidity. Nitrogen gas would be the recommended option.
BGA components should be used after 8 hours from pack opening. Ii is a common failure in the process to exceed this time limit. Baking temperature used is around 125°C. A lower temperature will not achieve correct dehumidification, while higher temperature than needed could affect metallographic structure between solder balls and components
Stencils are made of stainless material, their thickness, aperture sizes and the use of frame or non-framed stencils is very important to ensure the proper and accurate dispensing of solder paste onto the board. stencil thickness should be limited within the common range from 0.12mm to 0.15mm, and laser cutted.
Too much paste could create shortcuts in between fine-pitch BGA balls and too little paste insufficient wetting and cold solder joints. Balancing the wetting by ensuring sufficient flux is needed. Pressure range will go from 35N to 100N and printing speed from 10 mm/s to 25 mm/s
Is essential in this process not only the quality of course but also the correct particle diameter should be chosen. Regarding quality we look forward to excellent printability and solderability, also less contaminant.
Solder particles need to be coherent with the pad and lead size. We could think that smaller the pitch smaller the particle but is not always so lineal this relation and particular considerations will be done in each case. As general recommendation solder paste below 45μm particle diameter will cover both needs
BGA components placement and mounting
Accurate mounting here is criticall, although solder balls would self center we need to complete this operation with high precision. BGA/CSP rework station and chip mounter is used for this, precision of chip mounter reaches approximately 0.001mm. Solder can be inspect, searching coplanarity defect and recognize some other defects such as missing balls. Local fiducial marks are set or a couple of fold lines are set as fiducial marks for manual inspection after assembly.
Going further in guarantee solderability, BGA components can be controled by 25.41μm to 50.8μm by height, also we applied during 400 ms a delay shutdown vacuum system. This way solder balls and solder paste contacts together and void soldering of BGA components can be decreased.
This is the most difficult phase to control, also a dificulting issue to attend is that BGA reflow temperature curves are not exactly the same in SMDs tan in BGAs. Temperature curve setting is crucial in the soldering joints forming process. So this would be something to really take care off.
After soldering, process includes a rework station. Here each chip can be reworked independently ion so that the BGA components can never be used again once they are disassembled from circuit board. A hot air reflux nozzle with the right size is used to cover the BGA area without affecting the surrounding components
Different type of solder defects could appear. An open solder joint could be the result of insufficient temperature during reflow. This is because the existence of a non-collapsed ball. Also we could have intermittent connections, known as BICs (BGA Intermittent Connections). This will cause a aleatory failure very hard to detect once the PCB is fully assembled. Balls could be cracked causing short circuit or open circuit.
X-ray inspection in BGA technology
|Since the joints are not on the surface, another method is necessary to guarantee quality, so X-ray technologies are applied. 2D inspections searches for cracks, bridging, bad alignment or also insufficient solder. this is the low cost option. 5D X-ray solution will also compare the inspected PCB with the CAD file.We can analyze three individual slices between the BGA and solder balls, also get inside the solder balls and deeply analyzes the connection between the balls and the pad, Thus, our engineers can find flaws that with another technique would be impossible.|
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