Choosing the right finish

I’m currently working on a 4-layer PCB design with FR4 material and a thickness of 1.6mm. I have ICs with 0.5mm pitch pads and some very small LEDs with 0.4mm pitch pads. I’m debating between using ENIG (more expensive) and HASL finish. I’ve read that for BGAs, ENIG is crucial, but I’m unsure about the pitch limit for QFNs where HASL finish would suffice. Additionally, does the choice between leaded or non-leaded solder make a difference in this context?

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ENIG is definitely the best finish for this application. It produces a thin, even coating which is necessary for mounting components this small and has excellent solderability. By comparison, Hot Air Solder Level produces a thick, uneven coating that doesn’t work well at all with fine pitch components. Pitch limit for HASL is considered to be at least 0.8mm but the process is so inconsistent and varies by machine that it’s best to run an F/A. Aside from environmental issues lead and lead-free both have pros and cons but both get the job done. Leaded has a lower melting temp, is easy to work with and a bit cheaper but some believe lead-free forms a better bond as it doesn’t flow as much. Think the main difference is how someone fells about lead.

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There are several advantages to opting for ENIG over HASL. Often, I’m willing to spend a bit more on ENIG, even during prototyping (although pricing can vary between fabs), because it significantly simplifies achieving proper solder connections, especially on components with small gaps.

Typically, your fabricator can provide specific guidance on when HASL might bridge between islands (i.e., minimum pin-pin clearance for HASL). The critical reason for choosing ENIG with components like BGAs is the exceptional flatness it offers compared to HASL. With HASL, there’s a concern about ensuring all BGA balls sink onto the pads evenly and quickly; uneven connections can lead to instability or misalignment. ENIG reduces this risk by a considerable margin.

Most of the PCB fabs I know recommend a minimum lower limit of 10 mil (0.254 mm) for HASL. However, for assembly houses that handle both PCB fabrication and assembly, they generally prefer ENIG or similar finishes for pad clearances of 15 mil (0.381 mm) and below. Sometimes, they advise ENIG even for clearances below 20 mil, particularly with large TQFPs (>160 pins), to mitigate any skewing risks. However, many TQFPs already have pin clearances of 15 mil or less. By designing connections with appropriate thermal reliefs and neckdowns, the risk of issues during assembly becomes minimal.

Another benefit of choosing ENIG, regardless of component type, is its effectiveness with thermal pads on TQFPs. The flatness of ENIG makes it easier to achieve clean, uniform solder flows on these pads, although similar results can often be achieved with HASL.

In my experience, if I’m dealing with very small components (<=0.5 mm pad-to-pad spacing) and want to avoid potential issues during production or assembly, ENIG is a worthwhile investment. Alternatively, if wave soldering isn’t a concern, leaving an area uncovered by solder resist can also yield a durable and aesthetically pleasing gold finish.

Overall, ENIG offers robust advantages in terms of soldering reliability and component placement accuracy, making it a preferred choice for many PCB designs, especially those with critical component clearances and thermal considerations.

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