What is deflux cleaning? Stations and inline cleaners

Summary: what is deflux cleaning?

Front end or back end?

Back end. Semiconductor manufacturing has two main stages: the front end builds circuits on wafers in the fab; the back end, at the OSAT, cuts wafers into dies, packages them into finished parts that can be soldered to circuit boards, then tests and ships them. Deflux removes flux left after die attach and ball-mount reflow, and flux only appears in packaging's soldering steps, so it's a back-end process.

Two common points of confusion: BEOL (back end of line) refers to the metal interconnect process inside the fab, part of the front end, not packaging. Bumping and wafer-level packaging are sometimes called "middle end"; advanced packaging like CoWoS, though done inside fabs, is still packaging. This article classes all of these as back end.

Why clean off flux? What problems does residue cause?

Soldering needs flux to remove oxide from metal surfaces and help solder wet. After reflow, flux leaves residue. In flip-chip packages, the chip is soldered to the substrate with bumps; the gap between them is very low and the bumps dense, so residue gets trapped here most easily and is hardest to clean (Todd & Bixenman, Advanced Packaging).

The next step is underfill, which flows into the gap by capillary action and cures around the bumps to spread thermal stress. Residue causes three problems at this step (Henkel, Flux and Underfill Compatibility):

Even with "no-clean" flux, residue can still react with underfill materials; whether they're compatible depends on both formulations and can't be generalized. So high-end flip-chip packaging usually still includes deflux cleaning.

FCBGA flow: where cleaning happens

Taking flip-chip packaging (FCBGA) as the example (for the other 7 packages, see the 8 package flowcharts), the back end runs from wafer intake to packing and shipment in 55 steps. The wafer side and substrate side are two separate lines with materials from different factories. Each is loaded into frame cassettes or magazines when finished and sent to the assembly line, where they merge at flip-chip attach, followed by underfill, ball mount, singulation, and test. The diagram below is the FCBGA flow tree: the upper tree shows the wafer and substrate lines prepared separately; after they merge at "bonding," the lower tree shows the main packaging flow, with all steps listed under each station. There are 11 wet cleaning stations in all: 5 on the wafer line (pre/post bump plating, post-strip, post-bump-reflow, post-grind, post-dicing) are cleaned at the wafer stage on wafer tools; the other 6 are handled by the inline cleaners covered in this article, 3 for flux (deflux) and 3 for dust and mold flash.

The wafer side also has 5 wafer-level cleans (pre/post bump plating, post-strip, post-bump-reflow, post-grind, post-dicing), handled by wafer cleaning tools or the cleaning units built into grinders and dicing saws, plus 4 plasma cleans (2 depending on product). Actual stations and order depend on package type and each plant's flow. Station ③ is the most critical: it directly determines whether underfill fills properly. For packing and shipping, see the automatic semiconductor inner-box packing machine.

Why the cleaner follows the oven

An OSAT isn't one line running from start to finish. Wafers and substrates come from different factories and are prepared separately; each stage uses different carriers (wafers, substrate strips, single units) at different speeds, and some stations, like baking and testing, process a whole batch at once. So most stations move material lot by lot in magazines, with buffering in between.

The exception is a few fixed sequences with similar takt times, the inline segments (the blue box under the "bonding" station above): "flux → bonding → reflow oven → deflux" are linked directly by conveyor. Flux residue gets harder to clean the longer it sits after reflow, so cleaning right after the oven helps and saves a load/unload step. That's where the inline cleaner sits. When evaluating one, look beyond cleaning power to whether it can link to the reflow oven upstream:

Cleaning stations outside inline segments, such as dust removal after depaneling or dicing, are loaded and unloaded in magazine batches. For each of the 8 packages' inline segments and merge points, see Semiconductor packaging flowcharts.

Other packages?

No. Wire-bond packages (QFP, QFN) use no flux, so they have no deflux; WLCSP and fan-out do deflux at the wafer stage; CoWoS and CoPoS bond flip-chip twice plus ball mount, so at least three deflux steps, and the chip-to-interposer (CoW) step is the hardest to clean, with micro-bump pitch under 25 µm (ZESTRON).

Every step and cleaning station from wafer intake to shipment for all 8 packages is laid out in Semiconductor packaging flowcharts.

Inside an inline cleaner: 9 units

Packaging plants commonly use conveyor-type inline cleaners: substrates are placed on a mesh conveyor, pass through each unit in turn, and come out dry. The line divides into three zones:

1. Load

   An operator places substrates on the conveyor, or a loader hands them over. Once parameters are set on the control panel (PLC HMI), cleaning starts as soon as a substrate enters.

2. Chemical spray (1–2 stages)

   A pump pressurizes the cleaning chemical, which is sprayed onto the substrate from upper and lower spray bars. The chemistry dissolves the flux, and the spray's impact carries residue out of the gaps. The chemical is filtered and recirculated, and a pressure gauge shows spray pressure.

   Dissolve + impactFiltered loop

3. Chemical air knife

   Before the rinse zone, an air knife blows most of the chemical off the surface, reducing chemical carried into the downstream water tanks.

   Reduces drag-out

4. Overflow rinse

   Overflow water first rinses the remaining chemical off the substrate and drains directly; this water isn't recovered into the DI stages that follow.

5. DI water spray

   High-pressure deionized (DI) water rinses again, washing away remaining contamination and particles. DI water contains no ions, so it's less likely to leave water spots or conductive residue after drying.

   DI waterHigh pressure

6. Air knife

   Strong airflow blows most of the water off the substrate surface and out of the gaps.

7. Hot air knife

   Hot air then dries the remaining moisture, preventing water marks as it evaporates.

   No water marks

8. Unload & destatic

   At the unload end, an operator removes the substrates or they pass to an unloader. Substrates easily pick up static from air knives and conveying, so a static eliminator neutralizes them before unloading.

Why "pressure spray"? Fan nozzles and flow into the gap

Flip-chip gaps are very narrow; soaking alone doesn't refresh the chemical, so residue doesn't come out. The industry sums up cleaning effectiveness as four variables: chemistry, time, temperature, and impingement energy (Todd & Bixenman). Pressure spraying raises the impingement energy.

How fan nozzles work

Liquid under pressure enters the nozzle from the spray bar. The nozzle's elliptical outlet squeezes the liquid, and with nowhere else to go, the pressure spreads it sideways into a fan-shaped spray. A row of fan nozzles along a spray bar, with overlapping spray widths, forms an even curtain of water across the full conveyor width.

How water reaches the gap

As substrates move along the mesh conveyor, spray from above strikes at an angle, and the flow is driven into the gap between chip and substrate, pushing out the chemical and residue inside. Spray bars can be mounted above and below, so the substrate underside is cleaned too.

Why two air-knife stages?

An air knife is a long, narrow outlet that turns high-pressure air into a thin, strong stream that scrapes liquid off surfaces like a blade. Air knives appear in two places in a cleaner, for different purposes:

Air knives are often fed by a blower. After the drying stage, station ③ (after flip-chip attach) usually adds a bake to ensure no moisture remains in the gap before underfill.

Unseen but essential

5 questions before choosing a cleaner

1. Gap height, bump density?

   The lower the gap and denser the bumps, the more you need high-impact spray and chemistry suited to narrow gaps. Get the product's standoff height and bump pitch first.

2. Which flux is used?

   Water-soluble, no-clean, and rosin fluxes leave different residues, so the chemistry must match; check flux–underfill compatibility at the same time.

3. Which station?

   Deflux, post-dicing dust removal, and post-mold deflash need different unit combinations; not every station needs a chemical stage.

4. Throughput, substrate size

   These determine conveyor width and speed, and whether the spray section needs one stage or two.

5. How to prove it's clean

   Agree on acceptance first: visual or microscope inspection for residue and water marks, plus void inspection after underfill. Set acceptance criteria up front so you can compare options.

FAQ