Dynamic thermal deformation curves for commonly used BGA packages

Analysis of P-BGA and F-BGA thermal deformation process and stencil opening optimisation

In the field of electronic packaging, the thermal deformation behaviour of BGA (Ball Grid Array) packages has an important impact on package reliability and solder yield.The thermal deformation curve, as a key tool for describing this behaviour, visualises the deformation of the BGA package in three-dimensional space in the form of a two-dimensional diagram.The vertical coordinate represents the difference between the height of the BGA centre and the height of the corners or edges based on the measurement plane, i.e., the dynamic warpage (a positive value means the centre is convex, a negative value means the centre is concave or the corners are warped); the horizontal coordinate represents the temperature point.Understanding the thermal deformation process of P-BGA and F-BGA, the two most widely used BGA packages, is essential for optimising stencil opening design.

I. P-BGA thermal deformation process

The thermal deformation behaviour of P-BGA (Plastic Ball Grid Array) presents a typical two-stage characteristic, and its typical thermal deformation curve is shown in Figure 1-1:

Figure 1-1 Typical Thermal Transformation Curve for P-BGAs

Deformation mechanism before Tg point (BGA carrier board dominated)

When the temperature does not reach the glass transition temperature (Tg) of the EMC (encapsulated material), the coefficient of linear expansion (CTE, approx. 14ppm/°C) of the BGA carrier board (usually BT resin or FR-4 material) plays a dominant role.

Since the CTE in the plane direction of the carrier board is smaller than that in the vertical direction, the quadrangular region is the first to warp due to less constraints when heated, forming a negative dynamic warpage.

Deformation mechanism after Tg point (EMC dominant)

When the temperature exceeds Tg, the CTE of the EMC undergoes a sudden change (from 14 ppm/°C to 55 ppm/°C), and its volume expansion effect exceeds the carrier plate constraints, resulting in a reversal of the deformation direction to the centre upward convexity, forming a positive dynamic warpage.

This deformation reversal is exacerbated by the full-array distribution property of the solder ball array, making the centre region a stress concentration point.

II. F-BGA thermal deformation process

The thermal deformation behaviour of F-BGA (Flexible Ball Grid Array) is significantly affected by the Cu cap, and its typical thermal deformation curves are shown in Figure 1-2 (without Cu cap) and the relevant part of Figure 1-1 (with Cu cap):

F-BGA without Cu cap

The deformation trend is similar to that of P-BGA, but the upward warping of the corners is smaller due to the cushioning effect of the flexible substrate, and the amount of centre upconvexity is reduced accordingly.Its typical thermal deformation curve is shown in Figure 1-2.

Figure 1-2 Typical thermal deformation curve of F-BGAs

F-BGA with Cu cap

The Cu cap (CTE ≈ 17ppm/°C) reduces the free expansion space of the EMC by mechanical constraints, resulting in a 30-50% reduction in corner uplift.

The interface between the Cu cap and the EMC forms a stress buffer layer, the slope of the deformation curve is smoother around the Tg point, and the centre uplift is reduced by about 25% compared to that of the P-BGA.

III. Stencil opening design optimisation

Based on the thermal deformation curve characteristics of P-BGA and F-BGA, the following stencil opening optimisation strategies can be formulated:

P-BGA stencil opening design

Regional differentiation of the opening: According to the regional characteristics of the four corners of the upward curvature and the centre of the upward convexity of the opening design, respectively, the use of "shrinking" and "expanding" opening design, in order to compensate for the excessive or insufficient amount of solder paste.

Step Thickness Design: A combination of stencils is used to create a volume gradient of solder paste in the corners to balance the cooling contraction stresses.

IV. F-BGA stencil opening design

Cu-cap impact zone treatment: "Window" openings are used in the Cu-cap projection zone to take advantage of the flatness of the Cu-cap; "Dense-pitch micro-hole" design is used in the non-covered zone to enhance the uniformity of solder paste release.

Thermal stress buffer design: Stress relief slots are set up in the corners of the package corresponding to the stencil area to reduce the welding residual stress.

Co-optimisation and validation methods

Co-optimisation of temperature profile

By controlling the heating rate and peak temperature in the preheating zone, the thermal deformation behaviour of the BGA package is influenced, creating a synergistic optimisation effect with the stencil opening design.

Deformation Monitoring

Real-time acquisition of BGA surface morphology using laser interferometer to ensure deformation within a controlled range.

Solder joint reliability verification

The reliability and stability of the optimised solder joints are verified through shear strength tests and thermal cycling tests.

V. Suggestions for engineering practice

Material matching verification: Establish EMC-carrier plate-stencil opening parameter database, and configure corresponding opening schemes for different Tg values of EMC.

Intelligent stencil technology: Adopt variable thickness stencil (VPS) and achieve local thickness adjustment through laser engraving.

DFM co-design: Reserve BGA corners stress relief area in PCB layout stage, forming a double compensation mechanism with stencil opening optimisation.

VI. Conclusion

Through in-depth understanding of the thermal deformation process of P-BGA and F-BGA, and combined with the thermal deformation curve for the optimisation of the stencil opening design, the soldering yield and reliability of BGA packages can be effectively improved.The diagrams in the article can provide engineers with an intuitive visual guide, which helps to better understand and optimise the manufacturing process of BGA packages.In actual engineering, multiple factors such as material properties, temperature profile and stencil opening design should be considered to achieve the best package results.