SMT资料ProfileTroubleShooting英文资料

1、profiling & troubleshootinghttp:/ paste reflow the actual soldering process usually carried out using an oven oven may be infra-red or convection, the atmosphere air or nitrogen but any controllable heating method can workhttp:/ mount lineprinterpick & placereflow ovenhttp:/ & placehttp:/ ovenhttp:/

2、 reflow profile05010015020025015 3045 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300time (s)temp (0c)http:/ is an ideal profile? in most cases the profile is determined by the requirements of the board & components and the capability of the oven modern solder pastes will tolerate a

3、 wide variety of profiles however, certain aspects must be noted in order to get the most from the materialhttp:/ profile? soldering involves a chemical reaction in order to get consistent results the process must be monitored every board type has different thermal demands every oven behaves differe

4、ntly and may even change from week to week, day to day or hour to hourhttp:/ profile? statistical process control ensure consistency spot trends & adjust accordingly enable the use of max/min lineshttp:/ window (example)050100150200250300050100150200250300350time/stemperature/chttp:/ do i profile?ht

5、tp:/ do i profile? use a profiler such as a soldapro or a slimline soldapro oracle attach thermocouples to various areas of a scrap, populated (or part populated) board high density or heavy areas low density or light area sensitive components pass board & profiler through the oven then download to

6、a pc or printerhttp:/ components & thermocouplesusing hmp soldersoldaprolightly populated areaheavily populated areatemperature sensitivedevicehttp:/ through the reflow ovenhttp:/ (s)temp (c)board/smallcomponent largecomponentdownload to a pchttp:/ reflow profile0501001502002501545751051351651952252

7、55285time (s)temp (c)board/smallcomponent largecomponenthttp:/ guidelineshttp:/ heating ramp05010015020025015 3045 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300time (s)temp (0c)http:/ heating ramp object is to get profile to pre-heat stage limited by board & components (thermal sh

8、ock) explosive out-gassing from the solder paste will only occur under extreme conditions (eg laser soldering) common ramp-rates are 1-3c.s-1http:/ soak05010015020025015 3045 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300time (s)temp (0c)http:/ soak the pre-heat allows all areas of

9、 the board to reach a similar temperature before reflow with modern ovens it is possible to reduce or eliminate the “plateau” part of the profile there is no specific activation temperature for solder pastes. however the activators become more effective once they are molten or dissolved in molten re

10、sinhttp:/ soak rosins begin to soften and flow at temperatures between 70- 120c a simple law of chemistry says that reactions become faster at higher temperatures during the pre-heat (in aerobic reflow), oxidation will occur on exposed metal surfaceshttp:/ 3045 60 75 90 105 120 135 150 165 180 195 2

11、10 225 240 255 270 285 300time (s)temp (0c)reflow spikehttp:/ spike this is when the soldering occurs all areas must exceed the melting point of the alloy to allow powder particles to coalesce, wet the surfaces and fill capillaries a higher temperature will lower the surface tension of the solder an

12、d increase the efficiency of the fluxhttp:/ spike a higher temperature will increase oxide formation, discolour residues & board and may damge components a good reflow spike is usually about 30-40c above the liquidus of the alloy (eg. 210-220c for sn62) and will be above the melting point for 30-60

13、secondshttp:/ solidification05010015020025015 3045 60 75 90 105 120 135 150 165 180 195 210 225 240 255 270 285 300time (s)temp (0c)http:/ solidification rapid cooling will give bright shiny joints slow cooling allows more dissolution of substrates which will give rough or dull looking joints. taken

14、 to extreme, this can lead to de-wetting and poor strength the boards should be as cool as possible before being handled for health & safety reasons as well as ensuring that no joints are disturbed whilst molten http:/ cream reflow behaviourroom temperature- stable print- flux held between powder pa

15、rticles90oc- resins soften- solvent plasticises resin- gel structure begins to break downhttp:/ cream reflow behaviour150oc- solvent loss significant- resin melt very fluid- metal surfaces (powder and substrates) cleaned and re-oxidised rapidlyreflow (solder melting)- solder wets fluxed metals and f

16、ills capillaries. intermetallic is formedhttp:/ spread during reflow flux spread can carry powder particles away from the joint site flux spread can reduce the barrier against re-oxidation flux spread depends on :- resin blend plasticising solvents activatorshttp:/ favouring nitrogen reflow ultra-fi

17、ne pitch components use of low activity fluxes use of low residue fluxes non-ideal printing conditions poor dimensional tolerances variety of board & component finisheshttp:/ common reflow problems off - pad solder balling mid-chip solder balling tombstoning (manhattan effect) bridging (shorts) on f

18、ine pitch qfp leads open joints - usually fine pitch qfps dull or rough solder surfacehttp:/ - pad ballinghttp:/ - pad ballingreasons print misregistration excessive preheat oxidised solder pastesolutions adjust printer adjust reflow profile (increase belt speed or reduce pre- heat settings) fresh s

19、older pastehttp:/ ballinghttp:/ to mid - chip ballingdesign alter pad dimensions and/or shape thinner stencilprocess reduce placement height (not always possible) increase pre-heat soak (effective in marginal cases)http:/ solutions for mid - chip ballingd-shape ortriangular pads rather than square o

20、r rectangularhttp:/ bridging generally on fine pitch qfps often due to excess paste deposit or poor definition (smudged) depositshttp:/ bridging solutions thinner stencil narrower apertures - care to avoid aperture clogging more frequent under-stencil wiping adjust printer registration (print onto p

21、ad)http:/ solutions adjust placement machine (offset) increase pre-heat soak less active flux medium use 63s4 alloyhttp:/ tombestone pastehttp:/ components due to the use of smaller components such as 0402, 0201 and even 01005. placement (handling and positioning) and printing can become a problem.

22、printing can be overcome with good quality stencils and a good printer such as a dek (aswell as the right paste). poor placement how ever can lead to.http:/ of tombstone defectsgoodvertical tombstoneskewed tombstoneone termination wetted but no tombstonehttp:/ tombstoning paste wets one end of the c

23、omponent significantly before it wets the other end surface tension acts to lift the non-wetted end out of contact with the paste contributing factors different solderability of component terminations poor pad design poor component placement high activity flux poor activity fluxhttp:/ solutions adju

24、st placement machine (offset) less active flux medium. changing the design or process perameters.http:/ component tombstoning the best way careful pad design and component placement the “fix” modify reflow profile faster profile, or air nitrogen to prevent flux exhaustion slower profile or nitrogen

25、air to reduce flux activity hit and miss process change depending on specific circumstances process window improvement anti-tombstone pastehttp:/ anti-tombstone solder pastesslow down the wetting and solder flow processes to encourage component terminations to solder at the same time mixture of sold

26、er alloy powders with different melting temperatures sn/pb + sn/bi solder powder alloy with a melting range sn/pb/biboth techniques may leave solder joints with a significant melting range and reduced fatigue reliabilityhttp:/ anti-tombstone solder paste principle of operation unchanged uses mixture

27、 of alloy powders with very similar melting temperatures sn63 (183c) + sn62 (179c) final joint reliability not compromised small melting temperature difference not sufficient for good anti-tombstone effect uses mixture of powder particle sizes enhances anti-tombstone effecthttp:/ anti-tombstone solder pastes multicore powder size code “acp” mixed alloy powders (code 63s4) melting range 179-183c solidified alloy is sn63 with low ag (silver)“contamination” mixed powder sizes increas