--- title: Why HASL Surface Finish for Flexible Circuit Boards Is Not Recommended description: Learn why HASL surface finish is not recommended for flexible circuit boards and why ENIG is preferred for flatness, reliability, and processing. image: https://blog.epectec.com/hubfs/blog/fine-pitch-smt-with-hasl-surface-finish.jpg --- [![Epec Engineered Technologies](https://www.epectec.com/images/logo.png "Epec Engineered Technologies")](https://www.epectec.com/) [![Epec Engineered Technologies](https://www.epectec.com/images/logo.png "Epec Engineered Technologies")](https://www.epectec.com/) - [Articles](https://www.epectec.com/articles/) - [Blog](https://blog.epectec.com/) - [Webinars](https://www.epectec.com/webinar/) - [Videos](https://www.epectec.com/videos/) - [Case Studies](https://www.epectec.com/case-studies/) - [News & Events](https://www.epectec.com/news/) - [Customer Portal](https://system.netsuite.com/c.406760/site/NSlogin.html) - [About](https://www.epectec.com/about/overview/) - 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[Register](https://secure.epecpcb.com/epec/index2.jsp) [![](https://www.epectec.com/images/InstantPCBQuote-logo.png)](https://www.epectec.com/instantpcbquote/) [Login](https://secure.epecpcb.com/epec/) [Register](https://secure.epecpcb.com/epec/index2.jsp) Register to Quote and Order Your PCBs Fast, Easy & Online, 24/7 - [Home](https://www.epectec.com/) - [Blog](https://blog.epectec.com/) - Why HASL Surface Finish for Flexible Circuit Boards Is Not Recommended Share This Page: [LinkedIn](https://blog.epectec.com/#linkedin) [X](https://blog.epectec.com/#x) [Facebook](https://blog.epectec.com/#facebook) [Email](https://blog.epectec.com/#email) # Why HASL Surface Finish for Flexible Circuit Boards Is Not Recommended [![Paul Tome](https://blog.epectec.com/hubfs/Blog-Authors/paul-tome.jpg "Paul Tome")](https://blog.epectec.com/author/paul-tome) Written by [Paul Tome](https://blog.epectec.com/author/paul-tome) Posted on October 26, 2022 at 8:48 AM ![Paul Tome](https://blog.epectec.com/hubfs/Blog-Authors/paul-tome.jpg "Paul Tome") - [Tweet](https://twitter.com/share) [Flexible circuit boards](https://www.epectec.com/flex/) often require surface finishes that support fine-pitch components, reliable interconnects, and demanding manufacturing requirements. While hot air solder leveling (HASL) was once widely used throughout the electronics industry, it is now rarely selected for flexible circuit designs. Surface flatness requirements, thermal processing concerns, and handling challenges during manufacturing have made alternative finishes, particularly electroless nickel/immersion gold (ENIG), the preferred choice for most flex PCB applications. [![Check Out Our Free Guide on Rigid-Flex Design Flaws](https://no-cache.hubspot.com/cta/default/707152/ad159ff3-8f69-4b72-b825-e50205bae629.png)](https://cta-redirect.hubspot.com/cta/redirect/707152/ad159ff3-8f69-4b72-b825-e50205bae629) ## How the HASL Process Works The HASL surface finish is applied by immersing a printed circuit board into a pot of molten eutectic solder. After immersion, the circuit board passes between high-pressure hot air knives that remove excess solder and return it to the solder bath. Before solder application, the exposed copper features are cleaned using a micro-etch process to eliminate oxidation. A flux is then applied to promote proper solder wetting of all exposed copper surfaces. The parts may also be preheated to reduce thermal shock during immersion. After the HASL coating is applied, any remaining flux residue is removed through a cleaning process. HASL equipment is typically configured in either vertical or horizontal formats. Both perform the same function, although horizontal systems are generally preferred because they provide greater automation and typically produce more consistent surface topography. ![Circuit board BGA with ENIG surface finish](https://blog.epectec.com/hs-fs/hubfs/blog/circuit-board-bga-with-enig-surface-finish.jpg?width=800&name=circuit-board-bga-with-enig-surface-finish.jpg) *Circuit board BGA with ENIG surface finish.* ![Fine pitch SMT with HASL surface finish](https://blog.epectec.com/hs-fs/hubfs/blog/fine-pitch-smt-with-hasl-surface-finish.jpg?width=800&name=fine-pitch-smt-with-hasl-surface-finish.jpg) *Fine pitch SMT with HASL surface finish.* ## Surface Flatness Limitations in Flexible Circuit Designs One of the primary reasons HASL is no longer recommended for many flexible circuit applications is its inability to consistently meet modern flatness and coplanarity requirements. As solder is deposited and leveled by the air knives, the resulting surface is not as planar as other finishes. This variation can create challenges for components that require highly uniform solder pad geometry. Fine-pitch surface mount technology (SMT) devices and BGA packages often have tight coplanarity requirements that exceed what a HASL finish can reliably provide. Many [flexible circuit designs](https://www.epectec.com/flex/design/) also incorporate zero insertion force (ZIF) contacts. These contact structures require a highly consistent surface across the contact fingers, making HASL an unsuitable choice. Industry demand for flatter, more uniform surfaces contributed to the adoption of electroless nickel/immersion gold (ENIG), which addresses these requirements more effectively. ## Thermal Shock and Moisture Management Concerns The HASL process exposes flexible circuits to elevated temperatures as the circuits are immersed in molten solder. This creates additional manufacturing considerations that are not as significant with some alternative surface finishes. ![Flex circuit with stiffener delamination](https://blog.epectec.com/hs-fs/hubfs/blog/flex-circuit-with-stiffener-delamination.jpg?width=800&name=flex-circuit-with-stiffener-delamination.jpg) *Flex circuit with stiffener delamination example.* [Polyimide materials](https://www.epectec.com/flex/material-properties.html) are naturally hygroscopic and can absorb approximately 2% moisture by weight. Before HASL can be applied, this moisture must be removed through a controlled baking process. Depending on the layer count and specific design, baking can require between four and eight hours. If moisture remains trapped within the flexible circuit structure, exposure to the solder bath can cause it to rapidly convert to steam. This expansion can damage the circuit by causing coverlay separation, internal layer delamination, or other forms of structural failure. The [pre-baking requirement](https://www.epectec.com/flex/pre-bake-specifications.html) adds time and processing complexity while also increasing manufacturing costs through the use of large-capacity ovens and additional handling steps. ## Adhesive Materials Increase Delamination Risk [Flexible circuit constructions](https://www.epectec.com/articles/rigid-flex-pcb-material-and-construction.html) often include acrylic- or epoxy-based adhesives used to bond [coverlays](https://blog.epectec.com/what-is-a-flexible-circuit-coverlay) or join three or more circuit layers together. These adhesive systems typically have lower temperature tolerance than the polyimide materials used within the circuit. During HASL processing, the combination of elevated temperature exposure and improper handling can increase the risk of adhesive-related defects. Delamination can occur within the coverlay structure, between bonded layers, or around attached features such as stiffeners. This concern becomes more significant when using RoHS-compliant lead-free HASL processes because the higher processing temperatures represent a more severe thermal excursion than traditional leaded HASL. ## Manufacturing Challenges Unique to Flexible Circuits Rigid PCBs possess sufficient mechanical strength to pass through HASL equipment with minimal deformation. Flexible circuits present a different challenge. After emerging from the solder bath, the panel must pass between opposing high-pressure air knives that remove excess solder and establish the final surface finish. Thin flexible panels generally lack the rigidity required to remain stable during this process. To minimize movement, flex circuits often require support frames to hold the panel taut during processing. The framing process itself adds manufacturing complexity because panels must be mounted before processing and removed afterward. Even with support frames, very thin flexible designs may not remain sufficiently stable while passing between the air knives. Any panel movement or flutter can result in inconsistent solder thickness, reduced surface uniformity, and potential physical damage to the circuit panel. ## Why ENIG Has Become the Preferred Surface Finish The limitations associated with surface flatness, moisture management, thermal exposure, and panel handling have significantly reduced the use of HASL for flexible circuits. For many modern flex PCB designs, ENIG provides a more suitable surface finish because it supports the flatness requirements associated with fine-pitch SMT components, BGA packages, and ZIF contact structures while avoiding the processing challenges created by immersion in molten solder. As a result, demand for HASL on flexible circuits has declined to the point that many flex circuit manufacturers no longer perform the process in-house. When a HASL finish is required, some suppliers rely on third-party service providers rather than maintaining dedicated internal capability. When evaluating surface finish options for a flexible circuit design, the electrical requirements, component technology, contact features, and manufacturing constraints should all be considered to determine the most appropriate solution. ## Summary There are multiple reasons why a HASL surface finish either cannot be used on a flex circuit board design, due to the technology used, or at a minimum is not recommended due to the challenges it creates. This has significantly reduced demand, to the point where many flex circuit suppliers no longer support the HASL process in-house and, if absolutely necessary, will use a 3rd-party service company to apply the HASL finish. The ENIG surface finish has become the standard for flexible PCB designs. Please feel free to [contact Epec to discuss your requirements](https://www.epectec.com/flex/info-request.html) and to evaluate the surface finish options available for your specific design. --- ## Key Takeaways - **HASL is incompatible with modern flex PCB demands:** The uneven surface topography created by HASL doesn't meet the flatness and coplanarity requirements of fine-pitch SMT components, BGA packages, or ZIF contacts, making it unsuitable for precision applications. - **Thermal shock poses a serious risk:** Flex circuits must be thoroughly baked before HASL to remove absorbed moisture from polyimide materials. If not, the high-temperature immersion in molten solder can lead to delamination and permanent damage. - **Material limitations amplify processing challenges:** The adhesives used in flexible PCBs are more susceptible to heat damage than rigid boards, increasing the risk of defects during HASL application, especially under the higher temperatures required for RoHS-compliant lead-free solder. - **Handling during HASL is difficult for flex circuits:** Flexible PCBs lack the mechanical rigidity needed to pass cleanly through high-pressure air knives without warping or fluttering, leading to uneven solder thickness and potential panel damage. - **ENIG is the preferred surface finish for flex designs:** Due to its superior flatness, compatibility with fine-pitch components, and gentler processing requirements, ENIG has become the go-to surface finish for most flexible circuit board applications. - [Tweet](https://twitter.com/share) Topics: [Flex & Rigid-Flex PCB's](https://blog.epectec.com/topic/flex-rigid-flex-pcbs) --- [Next](https://blog.epectec.com/solid-works-configurations-for-parts-and-assembly-modeling) | [All posts](https://blog.epectec.com/all) | [Previous](https://blog.epectec.com/flexible-circuit-boards-with-lap-joints-design-considerations) [![New Call-to-action](https://no-cache.hubspot.com/cta/default/707152/b8c303b3-e840-4069-8d8e-14062c4c1260.png)](https://cta-redirect.hubspot.com/cta/redirect/707152/b8c303b3-e840-4069-8d8e-14062c4c1260) ## Leave a Comment --- ## Related Posts --- ### Recent Posts --- ### Posts by Topic - [Printed Circuit Boards (161)](https://blog.epectec.com/topic/printed-circuit-boards) - [Product Design (138)](https://blog.epectec.com/topic/product-design) - [Battery Packs (121)](https://blog.epectec.com/topic/battery-packs) - [Cable Assemblies (102)](https://blog.epectec.com/topic/cable-assemblies) - [Flex & Rigid-Flex PCB's (95)](https://blog.epectec.com/topic/flex-rigid-flex-pcbs) - [Electronics Industry (61)](https://blog.epectec.com/topic/electronics-industry) - [User Interfaces (55)](https://blog.epectec.com/topic/user-interfaces) - [Quality Solutions (33)](https://blog.epectec.com/topic/quality-solutions) - [Flexible Heaters (30)](https://blog.epectec.com/topic/flexible-heaters) - [Events, Training, and Support (9)](https://blog.epectec.com/topic/events-training-and-support) - [RF Products (7)](https://blog.epectec.com/topic/rf-products) - [CNC Machining (3)](https://blog.epectec.com/topic/cnc-machining) - [Fans & Motors (1)](https://blog.epectec.com/topic/fans-motors) See All --- --- ### Need Help with A Project? 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