Preheating BGA components demystified

Anonymous (anonymous@undisclosed.example.com) — 2011-03-23T16:51:55+00:00

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	+	====== 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

brmlab user:sargon

Preheating BGA components demystified