When a design mixes surface-mount components with through-hole parts, the board's surface finish plays a bigger role than many engineers expect. ENIG and HASL are the two most common finishes, and both can work with DIP assembly, but they behave differently during wave soldering. Understanding how they interact with through-hole soldering helps you specify the right board and choose a DIP assembly service that can actually deliver reliable joints.
What DIP assembly involves
DIP (Dual In-line Package) assembly, also called through-hole plug-in assembly, is the process of inserting leaded components into plated holes on a PCB and soldering them in place. Typical parts include connectors, relays, transformers, electrolytic capacitors, terminal blocks, and other components that need strong mechanical retention. The process usually runs like this: component forming and insertion, wave soldering, lead cutting, repair welding, board washing, and functional testing.
Wave soldering is the heart of DIP assembly. The board passes over a wave of molten solder, and the solder wicks up through the plated holes to form a joint around each lead. Because the solder must flow through the hole and wet the pad and lead together, the surface finish on the board directly affects how cleanly that joint forms.
How ENIG and HASL behave in through-hole soldering
HASL (Hot Air Solder Leveling) coats the copper with a layer of solder. For through-hole work it has a real advantage: the thick, pre-tinned coating on the plated hole walls gives the wave solder plenty of material to bond with, which supports strong, well-filled joints on connectors and other leaded parts. It is also the most economical finish, which matters when a board is cost-sensitive. The trade-off is surface flatness. HASL leaves an uneven meniscus that can cause problems on fine-pitch SMT pads, and it has a shorter shelf life before oxidation starts to affect wetting.
ENIG (Electroless Nickel Immersion Gold) deposits a nickel barrier layer followed by a thin gold layer. It gives a flat, uniform surface, long shelf life, and consistent wetting, which makes it the preferred choice for boards that also carry fine-pitch SMT parts such as QFN, BGA, or 0201 components. In through-hole soldering, ENIG pads wet reliably, and the flat surface reduces the risk of solder bridging in dense areas. The main consideration is cost, since ENIG is more expensive than HASL.
The good news is that both finishes are compatible with DIP assembly. The choice is really about what else is on the board and what your reliability and budget targets are.
How to choose between ENIG and HASL
Choose HASL when the board is through-hole heavy, the SMT parts are coarse-pitch, cost is the main driver, and the boards will be assembled soon after fabrication.
Choose ENIG when the board also carries fine-pitch SMT components, needs a long shelf life, or must meet high-reliability requirements in fields such as medical, automotive, or communication equipment.
If a design mixes fine-pitch SMT pads with large through-hole connectors, ENIG on the whole board is often the safer call, because the flat surface protects the fine-pitch side while through-hole joints still form cleanly.
The practical point is to state the finish clearly on your order and confirm that your assembly partner can handle both. A factory that runs SMT and DIP lines together can give you a straight answer on how the finish will behave in wave soldering before you commit.
What to look for in a DIP assembly service
Dedicated through-hole lines: a supplier with dedicated DIP plug-in lines and wave-soldering equipment can control the process rather than treating it as an afterthought.
Process discipline: look for a documented flow from component forming and insertion through wave soldering, lead cutting, repair welding, board washing, and functional testing.
Inspection and testing: plug-in AOI, visual inspection, and functional testing catch defects before boards ship.
Quality systems: certifications such as ISO 9001, ISO 13485, and IATF 16949 indicate a controlled manufacturing environment.
Finish compatibility: confirm the supplier supports both ENIG and HASL, and can advise on which one suits your board.
Working with a one-stop partner
Farway Electronic is an electronics manufacturing services provider in Shenzhen that covers the full chain from
pcb board making to SMT, DIP, coating, testing, and finished-product assembly. For through-hole work, it operates two DIP plug-in production lines with two wave-soldering machines, 24 rear-welding stations, and a board-washing machine, supported by plug-in AOI and IPQC and QA sampling. The DIP process runs from component forming and insertion through wave soldering, lead cutting, repair welding, board washing, and functional testing.
On the surface-finish side, Farway supports lead-free HASL, OSP, ENIG/immersion gold, electrical gold, immersion tin, and immersion silver, so you can specify ENIG or HASL and get a board that matches your design. Its quality system covers ISO 9001, ISO 13485, IATF 16949, and ISO 14001, and assembly follows the IPC-A-610 standard.
Whether your board is a simple through-hole prototype or a mixed-technology design with fine-pitch SMT and DIP parts, the right partner will help you pick a finish that assembles cleanly and holds up in the field. A capable
dip plug-in welding service should be able to run your board on either ENIG or HASL, so it pays to confirm the whole chain before you order.
