When a PCB includes BGA components, the BGA area is often given a different surface finish from the rest of the board. The main reason is to balance BGA solder-joint reliability and assembly yield against overall board cost and the process window required by other areas. A common approach is to use ENIG or ENEPIG in the BGA area and OSP elsewhere. In some consumer products, the opposite approach is adopted: OSP is used in the BGA area to avoid gold embrittlement and potentially improve drop reliability.
How BGA Pads Differ from Standard SMT Pads
Hidden and Difficult-to-Repair Solder Joints
After BGA soldering, the joints are located underneath the package. They cannot be inspected directly by AOI, and ICT may not detect every defect. If a joint has insufficient soldering, head-in-pillow defects, or cracking, the usual remedy is to remove and replace the entire BGA. In severe cases, the complete PCB may need to be scrapped.
Fine Pitch and High Coplanarity Requirements
For BGAs with a pitch of 0.8 mm or less, both the pads and solder balls are relatively small. These packages are highly sensitive to pad flatness, coplanarity, and solder-paste printing accuracy. Even small variations in pad height or solder volume can affect the formation of individual joints.
Small Solder Volume per Joint
A small solder volume increases the influence of intermetallic compounds (IMCs), gold embrittlement, voiding, and poor wetting on individual-joint reliability. A defect that might be tolerable in a larger conventional SMT joint can become a significant reliability concern in a fine-pitch BGA joint.
Stress Concentration
BGA packages are often relatively large. During thermal cycling, PCB bending, or mechanical drop events, the greatest stress is commonly concentrated around the corner solder joints. The quality of the solder-pad interface therefore has a direct effect on package-level and board-level reliability.
For these reasons, a surface finish that is acceptable for ordinary SMT pads may not be suitable for BGA pads. This difference in sensitivity is the fundamental reason for applying a separate surface finish to the BGA area.
Key Considerations for a Separate BGA Surface Finish
1. Surface Flatness
HASL produces a solder-coated surface with variations in thickness and a characteristic uneven profile. This is unfavorable for fine-pitch BGA assembly because it can increase the risk of solder bridging, insufficient solder joints, and head-in-pillow defects.
BGA areas therefore commonly use ENIG, ENEPIG, OSP, immersion silver, or immersion tin. These finishes provide a relatively flat surface and are better suited to fine-pitch pads. Non-BGA areas may contain through-holes, large pads, or other features for which OSP or HASL is sufficient. Using a lower-cost finish in those areas can reduce total board cost.
In other words, the BGA area receives a finish with better planarity, while non-BGA areas use a more economical finish where the assembly requirements are less demanding.
2. Solderability and Wetting Uniformity
During BGA reflow, the solder paste and solder ball melt together. The pads must wet quickly and consistently so that the solder ball and paste form a unified joint. Nonuniform wetting can result in head-in-pillow defects, non-wet opens, or incomplete fusion between the solder ball and solder paste.
ENIG and ENEPIG generally provide fast and consistent wetting, which gives the BGA process a relatively broad operating window. OSP can also be used for BGA assembly, but its performance is more sensitive to film thickness, storage duration, the number of reflow cycles, and oxidation of the underlying copper. Its process window may therefore be narrower.
For this reason, PCB fabricators and assembly manufacturers often recommend ENIG or ENEPIG for BGA areas when consistent solderability is a priority, while using OSP in other areas to reduce overall cost.
3. IMC Formation and Long-Term Reliability
Because BGA joints are small, the IMC occupies a relatively large proportion of the joint structure. The type and thickness of the interfacial compounds can therefore have a substantial effect on fatigue life.
With an OSP finish, the copper surface reacts with solder to form compounds such as Cu6Sn5 and Cu3Sn. Copper can continue to diffuse during long-term exposure to elevated temperature. After extended thermal aging, the IMC layer may become thicker, and void-related reliability concerns may increase.
With ENIG, the soldering interface is primarily associated with Ni3Sn4. The nickel layer acts as a diffusion barrier, so IMC growth is comparatively slower and long-term thermal reliability can be improved.
ENEPIG adds a palladium layer. This structure helps prevent black pad, improves resistance to nickel corrosion, and can improve the strength of the soldering interface.
Consequently, automotive electronics, servers, communication equipment, and other high-reliability products often use ENIG or ENEPIG in BGA areas, while OSP or standard ENIG is used in other areas to control cost.
4. Black Pad and Gold Embrittlement Risks
If the ENIG process is not properly controlled, black pad can occur when the nickel layer is excessively attacked during immersion gold deposition. Because BGA solder joints are small, black pad can sharply reduce interfacial strength. In severe cases, an entire BGA package may detach, or the corner joints may crack.
Gold embrittlement is another consideration. The solder volume in a BGA joint is small, so the gold layer can dissolve into the molten solder more readily. If the resulting gold concentration exceeds the relevant brittleness threshold, brittle AuSn4 phases may form and increase the risk of brittle joint fracture.
When ENIG is selected for a BGA area, the gold thickness must therefore be tightly controlled. A typical recommendation in this context is approximately 0.05–0.1 μm. Excessive thickness can increase the risk of gold embrittlement, while an excessively thin layer may allow nickel oxidation and reduce solderability.
Some portable consumer products with demanding drop-reliability requirements may instead evaluate OSP for the BGA area. Because an OSP soldering interface contains neither gold nor a potentially brittle nickel interface, certain package structures may exhibit better mechanical-drop performance than they would with ENIG. This explains why ENIG is not automatically the best choice for every BGA application.
5. Multiple Reflow Cycles and Storage Conditions
If a PCBA uses double-sided assembly, or if the BGA must pass through another reflow operation later in the process, the BGA pads must withstand multiple high-temperature cycles.
After repeated reflow, the organic OSP film can decompose, leaving the copper surface more vulnerable to oxidation and reduced solderability. ENIG and ENEPIG are generally more stable under repeated thermal exposure and can maintain better solderability after multiple reflow cycles.
When the assembly sequence is long, the number of reflow cycles is high, or humidity control is less consistent, using ENIG or ENEPIG in the BGA area can provide additional process margin.
6. Cost and Manufacturing Feasibility
Applying ENIG or ENEPIG to the entire board increases cost, particularly for large PCBs. If the non-BGA areas contain only standard SMT pads or through-hole pads, OSP may be sufficient and full-board gold plating may not be necessary.
A typical selective-finish process is to protect the non-BGA areas with dry film after solder mask processing, apply electroless nickel and immersion gold to the exposed BGA pads, and then strip the film and clean the board before applying OSP to the rest of the copper. The BGA pads that have already received ENIG are not coated with OSP, or receive only negligible additional coverage that does not affect soldering.
This approach can preserve BGA soldering quality while reducing the amount of gold chemistry required. However, selective processing adds dry-film lamination, exposure, development, stripping, and cleaning steps. The PCB manufacturer must have adequate process capability. Otherwise, alignment errors, chemical attack on the solder mask, or OSP contamination of the gold-finished pads may occur.
Common Combinations of Localized Surface Finishes
ENIG on BGA Pads and OSP on Non-BGA Areas
The BGA area receives a flat finish with good wetting behavior and resistance to multiple reflow cycles, while OSP provides adequate solderability at lower cost in non-BGA areas. This combination can significantly reduce the cost compared with applying ENIG across the entire PCB.
It is commonly considered for consumer electronics, industrial control equipment, and communication products.
ENEPIG on BGA Pads and OSP or ENIG Elsewhere
When the BGA is considered a high-reliability soldering area, ENEPIG provides a stable interface and avoids the black-pad concerns associated with conventional ENIG processing. OSP or standard ENIG can be used in non-BGA areas to manage cost.
This combination is suitable for applications such as automotive electronics, servers, base stations, high-layer-count PCBs, and other products with demanding reliability requirements.
OSP on BGA Pads and Another Finish Elsewhere
Because BGA solder balls contain a small solder volume, the risk of gold embrittlement associated with ENIG may be relatively significant. Under drop impact, an OSP interface may provide better resistance to mechanical stress for certain package structures. This approach also offers a very low board cost.
However, it requires tighter control of OSP film thickness, storage time, and the reflow profile. It is therefore not preferred by every PCB fabricator or assembly manufacturer.
Why Is BGA ENIG with Non-BGA HASL Uncommon?
HASL immerses the board in molten solder, making it difficult to apply selectively. If HASL is performed first, the pads are already covered with solder and cannot subsequently receive localized ENIG. If localized ENIG is applied first and the entire board then undergoes HASL, the high-temperature soldering process can damage the ENIG surface.
As a result, mixed surface finishes are more commonly implemented with selectively depositable wet processes such as OSP, ENIG, ENEPIG, immersion silver, and immersion tin. HASL is generally unsuitable for this type of localized combination.
Conclusion
Localized surface finishing in a BGA area is fundamentally driven by the higher sensitivity of BGA joints to pad flatness, solderability, IMC formation, gold embrittlement, black pad, multiple reflow cycles, and long-term reliability.
A single finish across the entire board simplifies manufacturing, but it may not simultaneously satisfy the requirements for high BGA reliability and low cost in non-BGA areas. For this reason, ENIG or ENEPIG is commonly selected for BGA pads, while OSP or another economical finish is used elsewhere. In products that are especially sensitive to drop reliability or gold embrittlement, OSP may instead be selected for the BGA area.
The choice should be evaluated according to the product reliability class, assembly sequence, BGA pitch, solder-ball alloy, cost target, storage conditions, and the PCB manufacturer's selective-processing capability. A mixed surface finish is therefore not an arbitrary layout choice; it is a manufacturing and reliability decision that must be validated against the complete assembly process.
