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Glossary for BGA.NET

Attach balls to BGA – sometimes required to convert solder ball composition from lead-free to tin-lead or vice versa.  Three steps are involved:  (1) removing the ball(s), (2) replacing them, and (3) applying reflow.  

Automated X-ray inspection  (AXI) – with BGA connections underneath the chip and not visible, AXI is critical for locating defects such as opens, shorts, missing parts, mis-aligned components, as well as insufficient or excessive solder.  

Ball grid array or BGA  – among the most popular chip packaging technology chipmakers use due to its high interconnection density and the less PCB real estate it takes.  In contrast to early package lead connections, a BGA as the name implies uses literally thousands of tiny balls underneath the package to connect to the substrate or PCB.  These solder balls or bumps convey electrical connections to the outside of the BGA.

Balls – also referred to as spheres.  A ball grid array can have a range of 500 to upwards of 1000 spheres or tiny metal balls organized in an orderly array of rows and columns.  They serve as the electrical connections from a chip like a µP to the PCB with those electronic signals going to other system points and connections.

BGA placement – advance pick and place systems are required for placing BGA accurately and other device placement on a PCB during assembly process. Some of the well known placement systems in the industry are FUJI, MyData, Siemens and JUKI.  

Column grid array or CGA
– similar to a BGA, but in this case, instead of solder balls, high temperature solder cylinders or columns, arranged in a grid pattern, make the electrical connections to the PCB.  A popular type uses ceramic for its body and is called ceramic column grid array or CCGA package.

Conformal coating
–  can be of different methods  depending on the application.  The common purpose among the different ones is to protect circuitry against moisture, dust, chemicals and temperature extremes.

De-populate – in some cases, the BGA may need to be de-populated to correctly place the BGA again, since it didn’t get soldered properly in the first attempt. There could be other reasons for de-populating the BGA’s, sometime to debug the boards, sometime to replace the Eutetic BGA to Lead Free and vice-versa.

Eutectic  – is another term for “leaded,” meaning lead is used in a BGA package or a PCB for example.  

Flip chip – a way of connecting chips to external circuitry via solder bumps deposited on the chip pads located on a semiconductor wafer’s top side.  The chip is flipped over when it is mounted to a PCB so that its topside is down, hence flip chip.   

Lead-free – means a BGA or other type of device package or PCB sub-system does not contain any lead, as per RoHS directives.

MEMS – stands for micro-electro-mechanical systems.  A MEMS device combines conventional silicon-based circuitry and silicon-based micro-mechanical structures.  Both electro and mechanical functions are based on traditional and cost-effective CMOS processing.  Growing numbers of PCBs are populated with MEMS and BGA packaged devices.

Paste height inspection – after printing PCB with solder paste, its inspection is critical to ensure that the proper amount of solder paste has been placed on the SMT pads, using a laser to measure the paste height. This ensures high yields, minimize rework/retest, and reduce overall scrap costs.  

Reworking BGA – could mean simply mean that you are reflowing BGA second or third time for proper ball collapse which did not occur the first time or it could mean removing and replacing BGAs on a PCB. Rework could also mean that you are de-populating BGA, re-balling it and populating it again on the board.

Solder paste types – 3, 4, and 5 
– JEDEC and standard J-STD 005 classify solder paste based  on the size of metal particles and mesh size in lines-per-inch comprising the paste.  Type 3 features -325/+550 mesh size and particle size of 45-20 micrometers (µm), Type 4, -400/+635 mesh size and 38-20 µm particle size, while Type 5, -500 mesh size and 25-10 µm particle size.
 
Stencil – a laser cut stainless steel foil with openings for every surface mount device to dispense solder paste on these SMT pads, prior to reflow. This solder paste printing process is very critical aspect of assembly process.  

Stencil design – depends on foil thickness and size of the aperture openings, both used to control the amount of paste to be deposited on SMT pads on the PCB.  

Stencil thickness – ranges roughly from 2 mils – 10 mils.  Correct thickness of a stencil is determined by the SMT device types to be placed on the PCB.

Thermal profile – can also be regarded as correctly calibrating the solder reflow oven to process a particular PCB project.  The profile plots temperature versus the time it takes for a board to move through the oven.  It is characterized by temperature, time, and heat transfer rate and is unique to each customer’s PCB order.
 
Underfill – is a polymer or liquid applied on the PCB after it undergoes reflow. It covers the fragile interconnects between the chip’s bottom side and the PCB’s top side.  Underfill provides extra rigidity with strong mechanical bonding between chip and the PCB’s corresponding connection.  

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