How to Control Vacancy Rates in BGA Soldering?

Introduction

In the PCBA manufacturing field, BGA (Ball Grid Array) soldering has long been regarded as a benchmark for process proficiency. Since solder joints are deeply embedded beneath the component, visual inspection cannot reliably assess quality—X-ray equipment is essential for inspection.

Although IPC standards permit a certain percentage of voids, for high-power, high-reliability electronic products, elevated void rates directly translate to reduced effective thermal contact area, diminished mechanical strength of solder joints, and even potential solder joint cracking during long-term operation. Controlling void rates is far more complex than simply lowering a single metric, it requires deep “fine-tuning” of the following five critical parameters.

I. Solder Paste Activity and Metal Powder Ratio

Solder paste serves as the raw material for soldering, and its chemical formulation directly influences bubble formation. During the PCBA placement process, excessive or insufficient flux activity can cause the flux to become trapped within the molten solder during evaporation. We generally recommend selecting solder paste with balanced synthetic resin content and strictly controlling the metal content.

For fine-pitch BGAs, Type 4 or Type 5 fine-powder solder pastes improve printing consistency. However, their increased surface area leads to higher oxide content, potentially raising voiding rates. Therefore, matching the solder paste specification to the BGA ball diameter is the primary defense against voids.

II. Reflow Oven Ramp Rate and Preheat Duration

The reflow soldering temperature profile is the primary factor determining void formation. During preheating, excessive heating rates cause solvents in the flux to boil violently, generating numerous microbubbles. Insufficient preheat time prevents the flux from fully deoxidizing before entering the melting zone, allowing residual substances to form pores within the solder balls.

Practical experience indicates that controlling the heating rate between 1.5°C-2.5°C/s and appropriately extending the dwell time between 150°C and 180°C allows volatile substances in the flux to escape fully. This “slow-cooking” strategy significantly reduces vapor pressure within the molten solder.

III. Time Above Liquidus

TAL determines how long the solder remains in a fluid state. In PCBA processing, if TAL is too short, bubbles may solidify before buoyancy can expel them from the joint. Conversely, excessively long TAL facilitates bubble removal but leads to thick intermetallic compounds (IMC), causing joint brittleness.

Finding this dynamic equilibrium requires precise measurement of the actual temperature beneath the BGA. Typically, controlling TAL between 60-90 seconds, combined with an appropriate peak temperature (usually about 30°C above the melting point), provides sufficient “escape” time for bubbles.

IV. Stencil Aperture Size and Thickness

Excessively large stencil apertures result in over-dispensing of solder paste by the printer, naturally increasing the total volatile content within the solder joint. For BGA pads, we often employ a reduction aperture design—such as shrinking the aperture diameter to 90% of the pad diameter or replacing circular apertures with square chamfered apertures.
This design alters the solder paste's accumulation shape, creating escape channels for flux gases during early reflow. This effectively prevents bubbles from becoming trapped within the solder ball core.

V. Dehumidification Pre-Treatment for PCBs and Components

Environmental humidity acts as a hidden catalyst. If PCB substrates or BGA components absorb moisture during storage, trapped water vapor instantly vaporizes under reflow temperatures. This sudden pressure surge not only creates extensive voids but can also cause severe “popcorn” effects, leading to package damage.

Before entering production line, professional PCBA factories must perform fundamental actions: subjecting components exceeding Moisture Sensitivity Level to forced baking at 125°C for 24 hours and dehumidifying PCBs.

Controlling void rates is a meticulous process experiment. If your product faces yield bottlenecks due to BGA cold solder joints or excessive voids, let us take a look. We will provide a process review tailored to BGA soldering and assist you in conducting comparative sample testing across multiple solutions.

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