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 ?

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

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 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

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 caltexsci website and wikipedia

 
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