How to apply underfill under BGA components correctly

Why is underfill used beneath BGA components?

Underfill is a low-viscosity epoxy that completely fills the gap between a BGA package and the circuit board. It spreads mechanical and thermal stress away from the individual solder balls onto the entire bonded area, which greatly extends the life of the joints.

Without underfill, every solder ball carries the load on its own. During temperature cycling, the difference in coefficient of thermal expansion (CTE) between component and board creates shear stress that fatigues the balls. Underfill mechanically couples both parts and reduces that load.

• Distributes shear stress from the solder balls across the whole area.
• Compensates the CTE mismatch between silicon, substrate and FR4.
• Protects against drop, vibration and moisture.
• Significantly raises the number of survivable temperature cycles.

Which underfill types are available?

Underfills are broadly split into capillary underfill (CUF), no-flow underfill and the reworkable variants. The choice depends on gap height, throughput and whether the component may have to be replaced later.

Reworkable underfill softens at roughly 150 to 190 °C so that the BGA can be removed again after local heating. Corner-bond attaches only the four corners, giving drop protection with minimal material and easy repair.

How is underfill dispensed and cured?

In the capillary method the underfill is applied along one or two edges of the BGA after soldering. The preheated board (typically 70 to 110 °C) lowers the viscosity so the material flows under the component by capillary action and fills the gap evenly.

• Preheat board and component to 70‑110 °C to improve flow.
• Apply material along one edge (L or I pattern) with a defined dose.
• Wait for capillary flow until a clean meniscus (fillet) forms on the opposite edge.
• If needed, add a second bead to build up the fillet.
• Cure per the manufacturer, usually 30‑60 min at 150 °C.

After curing, X-ray inspection confirms complete fill and the void level. A continuous, bubble-free fillet on all edges is the sign of a correct application.

Can an underfilled BGA still be repaired?

Only to a limited degree. Standard underfill is thermally stable and cross-links permanently, so removing the BGA usually destroys the component and often the pads too. Reworkable materials and corner-bond were designed specifically to preserve this repairability.

• Standard CUF: effectively non-repairable, high risk to pads and board.
• Reworkable: softens around 150‑190 °C, component removable.
• Corner-bond: release the corners and the part comes free - fastest rework.
• Always remove all residue, otherwise the new BGA will not sit flat.

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