How to determine automatic soldering machine solder joint defects?

Solder-joint quality decides long-term reliability of electronic assembly. The automatic soldering machine turns the hard-to-stabilize temperature, feed and alignment of manual work into reproducible actions, but defects still appear with process fluctuation. This article reverse-infers from ”symptom to cause”, based on IPC-A-610 ”Acceptability of Electronic Assemblies” and other public standards, sorting judgment points of four common defects-false solder, cold solder, bridge, solder balls-objectively and with evidence.

I. Objective criteria for a qualified solder joint

Welding essence is the metallurgical bond of solder and copper pad, lead at the interface generating intermetallic compound (IMC, typically Cu6Sn5, Cu3Sn). To judge qualified, see whether wetting is sufficient and IMC layer continuous. IPC-A-610 divides components by reliability into three classes: Class 1 general electronics, Class 2 dedicated service (e.g. laptop), Class 3 high-performance or harsh environment (medical, aviation, automotive), where Class 3 allows no cold solder. Public data: healthy IMC thickness about 0.5-2.0 microns, too thin or broken means insufficient bond.

II. False solder and poor wetting (non-wetting, weak wetting)

False solder shows as solder balling on the pad, base metal exposed, no bond formed. Causes: surface oxidation or contamination, insufficient flux activity, or temperature not reaching wetting. Such joints have low mechanical and electrical strength; per IPC-A-610 it is a defect, Class 2 and 3 both rework. Note: lead-free SAC305 normal joints are matte with slight grain-cannot apply the leaded ”shiny means good” standard, or easy misjudge.

III. Cold solder and disturbed solder joints

Cold solder often appears matte, granular or raised, the result of solder not fully melting and wetting. Cause one: insufficient heat conduction-large thermal-mass parts, thick copper layer absorb heat too fast, iron heat not reaching pad and lead; cause two: movement during plastic solidification (conveyor vibration, manual shake) forms cracked or wrinkled ”disturbed joint”. Both share missing or broken IMC layer, forming high-resistance brittle ”false connection” that may conduct short-term but fail long-term, and often passes initial power-on test.

IV. Solder bridge (Solder Bridge)

Solder bridge means adjacent conductors that should be electrically isolated are connected by excess solder; causes: too much feed, wrong tip geometry, too-tight pin pitch (e.g. 0.5-0.8mm fine-pitch devices). Bridge is defect in all classes, must remove. Common fix: solder wick by capillary action, avoid bare iron dragging that damages pads; root is controlling feed and tip selection.

V. Solder balls and spatter (Solder Balls)

Small solder balls scattered near the joint, mostly from flux outgassing, moisture, too-fast heating. IPC-A-610: acceptable only when encapsulated and not violating minimum electrical clearance, else defect. Control flux outgassing (e.g. solder breaking) and heating slope to reduce probability.

VI. Judgment method: from visual to cross-section

Visual inspection and AOI cover visible defects (cold-solder matte, bridge, solder balls); but BGA, QFN and other bottom hidden joints need X-ray (2D fast, 3D/CT more accurate); failure analysis uses cross-section plus SEM to observe IMC layer thickness and continuity. Note: electrical test finds open/short but not marginal joints, the latter being the field-failure hotspot.

VII. Prevention: start from the four elements

Defects mostly stem from the four elements ”heat, wetting, feed, stress”. Bolli 5331 dual-station soldering machine uses a self-developed control system linking high-frequency temperature control, solder-breaking machine (releases flux gas during feed, reducing spatter), auto-cleaning and teaching programming, reducing false-solder, cold-solder and solder-ball risk from the process end; its travel X axis 500mm, Y1/Y2 axis each 300mm, Z axis 100mm, max speed 500mm/s (customizable above 800mm/s), repeat positioning ±0.02-0.05mm, actual quality still subject to sample verification.

VIII. Summary

Solder-joint defect judgment must go from appearance, wetting and IMC bonding three layers, objectively handle by IPC-A-610 grade. Bolli on the 5331 dual-station soldering machine uses a self-developed control system linking high-frequency temperature control, solder-breaking feed and auto-cleaning, for 3C, automotive electronics, connectors and other precision soldering scenarios providing equipment and sample support; for specific process verification and selection communication, contact Bolli Automation Co., Ltd. 13352947677.