How to Choose Selective Soldering: Wave Pallet or Selective Soldering Machine — 5 Stages from Reviewing the Board to Leaving Design Clearance

When a board is mostly SMD with only a few through-hole parts, it cannot go over the wave as a whole, so the through-hole leads have to be soldered “selectively”. There are two common routes: cover the areas that must not touch solder with a masking pallet and run it over the wave, or use a selective soldering machine that solders joint by joint with a small nozzle. A masking pallet uses the wave machine you already have and gives high throughput, but one pallet usually fits only one design, parts that must stay dry need more distance from the openings, and preheat and wave height must be reset. A selective machine is more flexible, but it also needs clearance around the nozzle, and lead length is limited.

This article covers mixed through-hole and SMD boards: whether to solder the through-hole leads with a masking pallet over the wave or with a selective soldering machine, and what each route asks of the design. Selective machine model comparisons, parameter settings and flux selection are out of scope. There are 5 stages:

• Stage 1 | Why the board cannot go over the wave as a whole: mixed through-hole and SMD, bottom-side SMD that cannot see solder, panels that sag, through-hole parts on both sides.
• Stage 2 | Two routes: masking pallet over the wave, or a selective soldering machine.
• Stage 3 | Conditions and limits of masking pallets: one pallet per design, openings and clearance, preheat and wave height.
• Stage 4 | Design rules for selective soldering machines: clearance around the nozzle, lead length, board-edge clearance.
• Stage 5 | Choosing and verifying: volume, design stability, available clearance, then hole fill.

Each stage covers what to do, with the source in brackets, plus how JIAHON can help where relevant; sources, exceptions and tables are in the notes at the end. This is the general case; individual cases vary.

Stage 1 | Why the board cannot go over the wave as a whole

What to do:

• Mixed through-hole and SMD: most current designs are SMD, but some parts (for example buzzers, switches and connectors the user plugs and unplugs) cannot be made as SMD (source 3).
• Bottom-side SMD that cannot see solder: bottom-side SMD parts may not be wave-solderable, or, on boards built with double-sided reflow and no adhesive, may be washed off by the wave (source 3).
• Panels that sag: a very wide panel held only by the conveyor fingers may sag (source 3).
• Through-hole parts on both sides: use one pallet per side and two wave passes (source 3).
• List it first: put the through-hole parts to be soldered, the SMD parts around them and their distances into one table; both routes below use it (this article’s own arrangement).

Stage 2 | Two routes

What to do:

• Masking pallet over the wave: a pallet covers the areas that must not touch solder and leaves only the through-hole leads open to the wave (sources 2, 3).
• Selective soldering machine: a small-nozzle wave solders only where solder is needed (source 4).
• Which is better: a senior process engineer writing in an industry magazine says selective soldering is the ideal choice for mixed-technology boards, but not every plant has the equipment or the volume; masking the SMD with a pallet and wave soldering is a workable alternative with high throughput and short cycle time (source 2).
• Using existing equipment: masking pallets let a plant keep its current wave soldering equipment and process experience (source 3).

How JIAHON can help:

• Solder bar: the product page says solder bar is mainly used in wave soldering at electronics assembly plants and in hot-air solder leveling at PCB plants.

Stage 3 | Conditions and limits of masking pallets

What to do:

• One pallet per design: a masking pallet usually fits only one board, so the design should be stable before a large investment; a complete application usually needs no more than 10 pallets (source 3).
• Keep dry parts farther away: solder reaches the whole opening, so parts that must not be soldered need a larger distance (source 3).
• The pallet must seal: the pallet must sit fully against the board so solder does not leak into protected areas (source 3); leave enough clear area around each opening for the pallet wall to press on the board (source 1).
• Openings should not be too small: a paper hosted on the IPC website tested ground pins of memory module connectors in a lead-free process on a thick multilayer board: smaller openings gave more defects, and from 7.62 mm up the defect rate showed no difference; for signal pins and for tin-lead, opening size showed no effect. The authors concluded that a 7.62 mm opening is acceptable for most parts, provided the other design-for-manufacturing conditions are met; electrolytic capacitors may need a slightly larger opening (source 1).
• Match each opening with a keep-out: every opening size needs a matching SMD keep-out zone around the through-hole pad so the pallet wall can press on the board (source 1).
• Reset preheat and wave height: the pallet adds thermal mass, so preheat temperature must be adjusted; the solder has to flow into the pallet pockets, so the wave must be higher than usual (source 3).

How JIAHON can help:

• When a masking pallet has to be custom-made, JIAHON’s Process & Fixture Design page says that what standard products cannot solve, JIAHON will evaluate, design and build. Send photos or drawings, and JIAHON’s engineering department will reply whether it can be made and how.

Stage 4 | Design rules for selective soldering machines

What to do:

• Clearance around the nozzle: one company’s selective soldering design guide says that with a non-wettable nozzle, leave at least 1 mm from the nozzle’s outer edge to surrounding parts or SMD pads (source 4).
• Drag soldering needs more room: for drag soldering, keep at least 4 mm from the joint center to surrounding parts; with an allowed smaller nozzle, 1 mm is also possible (source 4).
• Lead length: for drag soldering, lead length is limited to 4 mm (the drawing says less than 4 mm); for dip soldering, 0.7 to 10 mm. Shorter leads bridge less; longer leads carry heat into the hole more easily (source 4).
• Do not let leads protrude too far: straight leads protruding more than 1 mm from the board usually do not take more solder or become stronger (source 4).
• Board-edge clearance: leave at least 3 mm along the top and bottom board edges for conveying and clamping (source 4).
• No SMD inside the solder area: inside a multi-nozzle solder area, SMD parts are generally not allowed unless already glued and soldered (source 4).

Stage 5 | Choosing and verifying

What to do:

• High volume, stable design, wave machine already in place: a masking pallet is usually the better fit (this article’s own arrangement, based on sources 2, 3).
• Frequent design changes, high mix and low volume: every design change means a pallet change, so a selective machine is more flexible (this article’s own arrangement, based on the one-pallet-per-design point in source 3).
• Crowded boards: when parts that must stay dry sit close to the through-hole parts, both routes need clearance; check the distances in stages 3 and 4 first, and if the room is not there, change the layout (this article’s own arrangement).
• Orientation helps, but it is not the only factor: in the paper’s test, one memory module connector perpendicular to the wave, whose ground pins connected to fewer ground planes, had better hole fill; but another comparison showed that heat from parts ahead of it could offset the disadvantage of running parallel to the wave (source 1).
• Verify: check through-hole fill, bridging and solder balls, and whether SMD parts next to the pallet or nozzle were affected (this article’s own arrangement).

Notes: sources and exceptions

The following is for readers who want to check; the steps above stand without it. Where your product specification, or your equipment or pallet supplier’s specifications, differ from this article, follow them.

Table 1 | Why selective soldering, and the two routes

Table 2 | Masking pallets

Source 1 figures come from a test on one thick multilayer board.

Table 3 | Selective soldering machines

The document calls itself a general guideline but also mentions the company’s machines; follow your equipment supplier’s specifications for the numbers.

• What was and was not read: four public sources (a paper on the IPC website, a senior process engineer’s magazine article, a research company’s magazine column, one company’s selective soldering design guide) and JIAHON’s Process & Fixture Design page. The full text of a selective soldering standard was not read.
• Where the sources stand: sources 2 and 4 name the same company on their pages, and the pages do not describe what kind of company it is; the page for source 1 does not describe the authors’ organization, venue or date; source 3’s author organization describes itself as a scientific research company.
• Figures are condition-specific: the 7.62 mm opening in source 1 comes from lead-free memory module ground pins on one thick multilayer board, with no opening-size effect seen for signal pins or tin-lead, and the authors add that electrolytic capacitors may need a slightly larger opening; source 4 calls itself a general guideline but also mentions the company’s machines. With a different board or machine, rely on your own verification and the equipment supplier’s specifications.
• This article’s own work: the list in stage 1, the choosing and verifying in stage 5 (volume, design stability, board density) and figures 1 to 3 are this article’s own arrangement; every other statement can be traced to a source or a JIAHON page.
• Translation: the sources are in English; statements here are paraphrased, not quoted.
• Scope: only the two routes for selective soldering and their design requirements; machine model comparisons, parameter settings and flux are out of scope.
• JIAHON’s help: “How JIAHON can help” covers only what JIAHON’s engineering department can assist with and wording already published on the site.

This is a general overview, not a restatement of any standard, and not an endorsement of any contract manufacturer, equipment maker or research company; and it makes no promise about your product or process. Go by your product specification, your equipment and pallet suppliers’ specifications and your own verification. This is the general case; individual cases vary.