A board with fine pitch SMT parts on the top side and a 60 pin connector through the hole cannot be soldered by one process without a decision. Wave soldering passes the whole panel over molten solder, fast and cheap per joint, but it heats everything already reflowed. Selective soldering brings a small nozzle to each joint, which protects the SMT side but adds programming time. Hand soldering is the third option and the one that costs programmes the most in field failures.
Pin count, panel space and how much heat the reflowed side can take settle the choice. Which machine is newer does not.
What each process actually does to the board
| Process | Fits best | Pallets and masking | Heat on SMT parts | Where it fails |
|---|---|---|---|---|
| Wave | Through-hole only boards, high pin counts, one sided THT | Pallets usually required, plus mask tape on open vias | Whole panel preheated and contacted | Shadows behind tall parts, bridging on dense pin rows, solder pulled onto the SMT side |
| Selective | Mixed technology boards, connectors near SMT, double sided THT | Pallets often not needed | Local, nozzle sized to the joint | Nozzle access blocked by tall parts, long programming for many points |
| Iron, manual or robotic | Prototypes, repair, a handful of joints | None | Local but uncontrolled | Fill varies with the operator, flux residue, cold joints on thick copper |
On a mixed board the wave has a second problem: solder wets the top side through an open via and lands on a reflowed pad. Pallets with cut-outs prevent it at the cost of a tooling charge per board type and a changeover at every panel change.
Decision rules that settle it before quoting
- Pin count. Under about 20 through-hole joints, selective wins on tooling and changeover. Between 20 and 200 pins on a mixed board, selective still wins because a pallet costs more than the programming. Above 200 pins on a board with no SMT on the solder side, wave remains the cheaper process per joint.
- Density. Pins on 2.54 mm pitch with tall electrolytic capacitors within 3 mm of the joint leave no room for a nozzle. That board goes back to wave with a pallet, or the layout changes.
- Double sided through-hole. When connectors are soldered from both sides, selective is the only repeatable route. A wave cannot do the second side without re-melting the first.
- Thermal mass. A 3.2 mm board with 4 OZ inner layers needs preheat that a mixed board cannot always take. Selective delivers heat where the joint is and leaves the rest of the panel alone.
The parameters that decide joint quality
| Parameter | Typical setting | What goes wrong outside it |
|---|---|---|
| Flux volume | Set per joint area, measured by weight on a test panel | Too little gives oxidation and icicles, too much leaves residue under a connector body |
| Preheat | Top and bottom zones, board surface 90 to 120 C before solder contact | Cold board pulls heat out of the nozzle and gives incomplete fill |
| Dwell time | 2 to 4 s per joint at SAC305 | Short dwell gives a cold joint, long dwell lifts pads and damages plastic bodies |
| Solder pot temperature | 255 to 265 C for SAC305 | Low temperature causes bridging, high temperature burns flux before it works |
| Nozzle size | Pin diameter plus 1 to 2 mm | Too small starves the barrel, too large wets neighbouring pads |
These are starting points. A 6 layer board with a ground plane on the solder side behaves differently from a 2 layer control board, and the dwell that fills one will not fill the other. The first article fixes the numbers.
Fill criteria to put in the drawing
Vertical fill is the number buyers argue about. IPC-A-610 accepts 75 percent barrel fill for Class 2 and Class 3 on most through-hole joints, with exceptions for specific pin types. A connector soldered on the far side needs its own note. Writing the class and the fill expectation into the drawing removes the argument at incoming inspection.
Fillet shape matters as much as volume. A concave fillet that wets pad and pin reads well on a photograph and can still hide a void in the barrel. X-ray on the first article, plus a cross section on one connector per panel type, turns a claim into evidence.
How SUPERB PCBA runs through-hole work
SUPERB Assembly runs wave soldering for through-hole boards and selective soldering for mixed technology, with manual and automated DIP insertion ahead of both. Panels are depanelled on an RS-500 cutter with optical alignment, so V-scored panels do not crack MLCC parts on the way out.
The SMT side that the through-hole process has to protect runs 0201 up to 55 mm BGA on a placer rated at 96,000 CPH. Reflow is a 10 zone oven with SAC305 lead-free as standard and nitrogen available. 3D SPI checks paste before placement, 3D AOI checks every visible joint after it, and 3D X-ray with CT covers BGA, QFN and LGA with void analysis to IPC-7095.
The workshop runs ESD S20.20 at 22 plus or minus 3 C and 45 to 65 percent RH, with ISO 8 conditions in the SMT areas. Workmanship follows IPC-A-610 Class 2 or 3 as the drawing states, against J-STD-001. SUPERB Tech quotes PCB and assembly together when panel and stack come from one programme, which keeps hole size, pad and soldering process in one conversation.
- PCBA workshop: line by line equipment, from paste printing to selective soldering.
- Class 2 and Class 3 explained: what changes for a through-hole joint.
- PCB capability: hole sizes, aspect ratio and copper weights that set the fill.
What the RFQ should carry
- Board thickness, layer count and copper weight on the through-hole layers.
- Pin count per connector, pin pitch, and the tallest part within 5 mm of a joint.
- Which side the connectors are soldered from, and whether both sides carry THT.
- Workmanship class and the vertical fill expectation per joint type.
- Panel drawing or array preference, plus the annual quantity that decides tooling.
Frequently asked questions
Can one board be reflowed and selectively soldered in the same line?
Yes, and it is the normal route for mixed technology. SMT is reflowed first, then the through-hole joints are soldered selectively. The order matters because the second pass must not disturb the first.
Does selective soldering remove the need for pallets?
On most mixed boards it does, which is where the saving comes from. A pallet is still needed when a board is thin and warps, or when the panel has open vias that would take solder from below.
Is pin in paste a replacement for both?
For a small number of joints it can be, and it removes a process step. It needs paste volume that fills the barrel, a connector body that survives reflow, and an aperture that does not starve the pin. Trial it before committing a connector family.
Next step
Send the Gerber data, the BOM and the assembly drawing with the pin count per connector and the class you need. The reply confirms which soldering route fits, flags joints that need a nozzle check or a pallet, and quotes with the inspection scope and fill criteria written in.
