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Don’t be surprise to know that there are more than 15 types of BGA and each BGA have different process parameters to be followed during the board assembly. Generally when we plan the printed circuit board assembly, we ignore to evaluate BGA placement capabilities of our EMS. We check the pick & place machine capability to mount BGA on our board but overlook to check the paste printing and reflow capabilities of that EMS. Below are some of the parameters to ensure the yield of assembled board with BGA remain consistent and above the industry standard.
PCB copper thickness & surface finish is one of the hidden attribute which affect the connection between ground pad and BGA silicon ball. It is recommended to go with ENIG(Electroless nickel immersion gold) or OSP(Organic Solderability Preservative) finish for BGA having pitch less than 0.6mm. Any other finish like Lead or Lead-Free HASL are with higher solder thickness on the BGA pad and results in to low assembly yield.
Stencil & Paste selection according to the BGA pattern and pitch is key requirement to get uniform paste deposition. Apart from this, the type of stencil you select is also important while you have BGA or multiple BGA in your design. Instead of Chemical etch stencil you should go with laser cut stencil process. Many of the street assembly vendors are not following the the right method and thickness of stencil. This will gives you low cost solution but most of the time yield loss cost is higher than the savings on assembly cost. To improve uniform printability for miniature components and small pitch BGA like Qualcomm Snapdragon series, type 4 and above paste types will definitely increase your yield. Water soluble, no clean paste and lead free solder paste also very useful to get higher assembly yield.
Vision system of pick & place is accurate on most of the Japan and German make machines. Fuji, YAMAHA and Siemens are pioneer in high accurate placement technology. While selecting your EMS you need to make sure that the machine make and models are not too old which can result in to inaccurate placement. Most of the BGA and components have self-centering / self-alignment property during solder reflow process. Along with this property if you maintain uniform paste thickness and accuracy in placement than your yield will be best above industry standard.
Thermal shock may damage internal circuits of BGAs and few reflow machines which are less than 4-5 chambers / zone can gives sharp thermal shock to the board. PCB material, component density and paste type derive the reflow profile setting for particular board. It is highly recommended that every board needs to undergo with minimum 3 sensors to set the reflow profile. One sensor at ball location of BGA, one at PCB copper surface area and one at any other component location. Many micro BGA are not qualified for more than 1 reflow cycle. Few TI and Broadcom processors are qualified up to 3 reflow cycles at peak temperature of 225 degree centigrade.
Small pitch BGA pad will have low strength and due to lower copper surface area of PCB pad generally one or many of the BGA pad moved out along with BGA chip. To avoid such pill off we have to use convective bottom side pre-heater to maximise temperature uniformity and encapsulated heating needs to apply on BGA such that the max heat reach to ball and solder joint area. Once you remove the BGA the site area needs to clean properly. In case of the same BGA need to remount on that surface you must have to recall the BGA to get desired performance of that BGA. More than 80% rework station are not with inbuilt or attached reballing facility. You can visually identify in x-ray that reballing is done or not after BGA rework. If the reballing is done than the pad density will be higher and looks all dots black whereas if reballing not done than the pads are on grey shade.