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--- user:sargon:preheating-demystify [2011/02/28 19:25] – sargon
+++ user:sargon:preheating-demystify [2011/03/23 16:51] (current) – sargon
@@ Line 1: / Line 1: @@
+====== Preheating BGA components demystified ======
+  * - What is BGA ?
+  * - What causes bad solder joints under BGA components ?
+  * - Types of bad solder joints
+  * - Is preheating solution ?
+  * - BGA soldering tips and tricks
+====== What is BGA ? ======
+[[http://en.wikipedia.org/wiki/Ball_grid_array|What wikipedia says ]]
+A ball grid array (BGA) is a type of surface-mount packaging used for integrated circuits.
+**YES !** ... it is a grid array of really small solder balls , each ball makes conductive joint
+between PCB and component packages
+solder balls under the package instead of leads
+{{:user:sargon:bga_package_sideview.png?200|}}
+====== What causes bad solder joints under BGA components ======
+BGA's are normally very stable and reliable components except components with a higher power consumption
+BGA components with higher power consumption have big problem with heat and material tension
+whole component is on solder balls which are not constructed to conduct heat from package
+and area where can they conduct heat is minimal .Rest of space under bga components is typically filled with
+air which does not conduct heat as well ...
+only way how to cool BGA components is heatsink on top of their package .And we have here another problem...
+We need sufficient area connected to heatsing if we want to cool whole package this mean that package is
+bigger and bigger packages are more sensible to [[http://en.wikipedia.org/wiki/Thermal_expansion|thermal expansion]]
+**Major issues ... heating , package size limitation , thermal expansion.**
+====== Types of bad solder joints  ======
+Because soldering is science by itself I want to write only about bad solder joints which are most common in post production ...
+BGA component is connection have 3 critical points
+  * connection to PCB
+  * connection to Package
+  * solder ball
+most common problem is cold solder joints or cracked joints
+Solder have one unpretty attribute, tin inside solder oxidates on air realy fast
+and as we know from elementary school oxide are not electrical conductor
+{{:user:sargon:bga_crack_1.jpg?200|}}{{:user:sargon:bga_defect.jpg?200|}}{{:user:sargon:bga_im8.jpg?200|}}
+====== Is preheating solution ?  ======
+In my opinion preheating or "reflow" does not help so much but can be used to diagnose failure or find which component is defective
+There are few facts why preheating/reflow BGA component cannot be used as a solution :
+  * we make additional huge stress to board and component
+  * oxidized joints cannot be soldered again
+but everybody can ask ... why preheating/reflow have such a big success rate ?
+during preheating physical connection can be "renewed" with applied heat
+but preheated solder is more vulnerable to cold solder joints or cracks
+====== BGA soldering tips and tricks ======
+I have some experience with soldering of BGA components and here are several tips ...
+Try to avoid "popcorn effect" ... what is popcorn effect ?
+Components and PCB can absorb humidity from air ... if we apply big heat on these
+parts humidity inside don't have enough time to evaporate and starts to make big pressure
+inside component layers or inside PCB ... this pressure causes mechanical damage of affected parts
+similar to popcorn ;)
+Tips :
+  * pre-warm board before reflow ( at 100 C for about 10-15 minutes )
+  * use profiled (programmable) heating gun / solder machine ( nothing opensource / openhardware available yet but I have some ideas)
+  * optimal heating slope is 5 Celsius per second during soldering state
+always use soldering flux paste ... there are several types from several manufacturers
+try to use under-heat ... when you are heating one side of the board
+opposite side is "colder" and this makes unwanted tensions inside board
+and can cause mechanical damage in board layers
+be aware of surrounding plastic parts ... you can cover them with thin Al foil
+**be aware of inhalating flux vapour/smoke ** it is highly toxic
+----
+pictures are from [[http://www.caltexsci.com/bga_scope.htm|caltexsci]] website and wikipedia