Design of reflow thermal profiles are very dependant on the components mounted on the board, and this specific application investigates the issue where two types of very commonly available BGA packages are mounted on the same assembly.
In the pictures below the two types are identified as a Plastic package BGA with an interposer layer thickness of 0.3mm and the other as a Laminate package BGA with an interposer thickness of 1.6mm.
| BGA : |
1.27mm pitch |
| Bump : |
Sn3.5Ag |
| Type-1 : |
Plastic package |
| Type-2 : |
Laminate package |
| PCB : |
FR-4, OSP, 0.6mm dia. pad |
| Pak temp. : |
230°C |
| Time above 220°C : |
> 30 sec. |
As the thermal demands on each type of BGA are different, thus the design of the thermal profile needs to be considered in order to ensure full melting of the balls on both types of component.
| |
220°C |
228°C |
230°C 220 sec. |
230°C 231 sec. |
| Type-1 |
 |
 |
 |
|
| Type -2 |
 |
 |
 |
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The above pictures show that due to the greater thermal mass of the type-2 BGA, approximately 11 seconds longer was required in the reflow soak zone of the oven in order to effect full melting and coalescence of the balls. It has also to be investigated if the type-1 BGA can withstand this thermal exposure for functionality after reflow. It can also be observed that if all the BGAs on the assembly were of type-1, then a slightly lower peak reflow temperature could have been used.
Below is a general guideline thermal profile for Koki S3X58-M406 series, our latest generation lead free pastes. This profile can be used in lead free assembly reflow and should cope with any lead free component mixture requirement, with a process window to suit the exact demands of the component mix actually used by the assembler. Note the reflow soak which is different to profile designs for tin lead based reflow soldering.
By Gordon Clark, Director Global Support
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