ABSOLUTE BASICS TO SUCH A SIMPLE PROCESS…

YEAH RIGHT

BUT… THIS IS SO SIMPLE… JUST ADD FLUX…

• By now you probably know better that many of the shortcut-eager quick-fixers will just give you a temporary fix but not a solution…

• So if really want to control your process, drive scrap down and bring your process under control maybe you want to start reading.

• In general a quick fix implies no thinking, and without analysis is like applying yourself a band aid to stop your neighbor’s bleeding.

UNDERSTAND YOUR PROCESS FIRST!!!

• There are many variables involved so make sure you UNDERSTAND FIRST

• Second… Analyze from the standpoint of requirements so you can figure out what you might be missing

REQUIREMENTS

• The surfaces to be soldered must be free of oxides and other contaminants in order to achieve proper physical and electrical bond

• The temperature difference between the unit to solder and the molten solder must not be so dramatic to produce CTE mismatch (product thermal cracking)

• Must have one (1) solder joint per pad/pin, no more (bridges) and no less (skips)

• No foreign materials present or undesired residues resulting from the process

• Components must be present, be the right component, on the correct position and properly connected

PARTS OF THE PROCESS/MACHINE

• Conveyor: as it takes the product across the different stages it is co-responsible for the results as it interacts with all requirements.

• Flux Applicator: Applies the solder facilitator. (simple!? )

• Preheating: Prepares Product and Flux for the arrival of solder.

• Wave Solder: the hot mean molten metal that we need to apply to the electro-mechanical bond points.

CONVEYOR SETUP PARAMETERS

• Inclination angle between 5°-7° (typically 6°) to control wave/pcb contact surface

• Transport speed determines EVERYTHING (fluxing, preheat profile, wave dwell)

• Leveling front-to-back determines homogeneous contact with flux & wave

• Parallelism affects board flatness and support.

CONVEYOR FINGER SETUP

• Different types of fingers

• V grooved fingers are to provide some pressure to hold PCB

• L-Shape fingers can have fingers “loose”

• Inverted spring tab keeps PCB from being lifted by the wave

• Critical: all fingers must be in-line

• Board must sit on top of the L and does not need to be pushed or held under pressure.

HOW TO SET UP CONVEYOR

• Leveling• In-to-Out 6°

• Front-Back Level Flat

• Clearance from bottom of PCB to:• Fluxer nozzle 2.5mm < Clearance < 8mm

• Wave nozzle 2.5mm < Clearance < 8mm

• Conveyor Width (L-Shaped) must be just sufficient to hold part from falling but no compression.

FOAM FLUXER SETUP• MUST be leveled also to produce homogeneous flux application

• Nozzle-to-board clearance

• Bubble Size homogeneous and as small as possible

• Air Supply dry and clean

• Controls for pressure and air volume are used to maintainbubble size and foam column height

• Air Knife setup critical• Avoids excess flux• Pushes flux up to top side pads

• PCB must not be dripping of flux when it leaves fluxer

UnleveledConveyor-fluxer

AIR KNIFE SETUP

• Angle too shallow or too much air• Collapses Foam Column

• Board underfluxed

• Angle too steep or little air• Excess flux on top (steep angle)

• Excess flux underneath dripping into preheaters (risk of fire)

CONTROLLING FLUX

• VOC / Solvent based flux evaporates at varying rates depending on temperature, atmospheric pressure and air flow

• Must keep constant control over solids content as solvent evaporates

• It will affect solder results, and residues• Foam consistency changes and alters bubble size.

• The lower the air flow the lower the evaporation, but will also lower foam flux column height.

• Maintain aerator wet in solvent at all times! Otherwise it will clog pores with solids

PREHEATERS

• Infrarred heaters (radiant)

• Heat bounces from metal shiny areas

• Dull areas (solder mask) raise higher temperatures

• Heater temperature >6x over PCB temperature

• Convection heaters

• Slower heat transfer rates

• Homogenous temperatures

• Heater temperature ~1:1 to PCB

PREHEAT SETUP

• Heat up ramp rates must meet Flux specifications (typically 2-3°C/Sec)

• Direct relationship with conveyor speed • faster speeds = lower ramp rates & low exit temperature

• lower speeds = higher ramp rates & higher exit temperature)

• Exit temperature must also be met. • Temperature too high or too soon before wave will consume flux before wave

• Temperature too low will leave too much flux on board (spatter, outgassing & solder balls)

• Temperature is evaluated at Top Side!!!

WAVE SETUP FOR ASYMMETRIC WAVE• Wave nozzle must be leveled

• Wave height (nozzle to top of wave)

• Minimize height to reduce dross

• Wave height must be at least 50%of PCB depth (relation to conveyor)

• Back flow must be minimal.

• Wave MUST be perpendicular to conveyor (consistency in height, penetration and contact dwell time)

SOLDER WAVE CHARACTERISTICS

• Front cascade must be laminar across PCB

• Flow around the edges of nozzle (front and back) is turbulent and will not ensure penetration and debridging

• Backflow speed must be equal to PCB speed to ensure debridging and excess removal

• Liquid temperature too low will also cause bad solder results

• Wave height above nozzle >2.5mm but as low as possible prevent component terminals from hitting the nozzle

USING GLASS TO VERIFY WAVE

• HIGH IMPORTANCE:• Preheat glass or it will shatter into pieces

• Conveyor must not compress glass

• Perpendicular contact area is relevant

• Cannot determine laminar flow you must ensure PCB is at least 6mm inside the nozzle edges

OTHER CONSIDERATIONS

• DFM • Pin to hole ratio desirable .008 to .020”

(0.2 to 0.5mm) for proper filling

• Lead to pad ratio >2:1

• Lead protrusion: shorter is better for component lifting reduction and solder bridging

• Topside pad reduction helps