培训教材FinePitchStencilPrinting钢板印刷与制程应对

1、fine pitch 鋼板印刷 & 製程應用http:/ of fpt printingn鋼板印刷的變數錫膏 ( solder paste)鋼板設計 (stencil design)印刷機(printer)刮刀 (squeegee)印刷參數 (printing parameters)http:/ (lead pitch)網目 (mesh)粒子尺寸 (particle size)n註:所得到的錫粒子,若其粒度大小分佈均勻適合時,要經由篩選過程。n例:角距為 16 mil 時,使用之網目為 type 4,3,錫粒子之範圍在 400/+500 之間。http:/ na. k.c.p.snb.

2、 m例: 90 3 m17n 金屬 錫粒子 濃度n 含量 type (黏度)http:/ sn63 / pb37 tmelt = 183o cnw / silver: sn62 / pb36 / ag2 tmelt = 179o cnno lead: sn96.5 / ag3.5 tmelt = 221o cnhigh temp: sn10 / pb88 / ag2 tmelt = 268o c - 302o chttp:/ pitch與錫膏使用關係lead pitchmeshparticle size 25 mil type 3 -325/+40025 miltype 3 -325/+400

3、 to 50020 miltype 3 -325/+50016 miltype 4,3 -400/+50012 miltype 4 -400/+625paste powder sizeshttp:/ mesh 尺寸ndesignation (um) (in) ntype1 (200) 740.0027ntype2 (250) 580.0023ntype3 (325) 440.0017ntype4 (400) 370.0015ntype5 (500) 300.0012ntype6 (625) 200.00078325400500(-325+400)astm :american society f

4、or testing and materialsmesh (網目):每一方寸內有多少錫球例:使用 type 3 角距為 20 mil ,其錫粒子為 1.7 mil (0.0017 in)錫膏顆粒尺寸http:/ rosin mildly activatednra:rosin activatednws / oa:water soluble / organic acidsnlr:low residue / no cleansrmas and ras do not typically have to be cleaned, but as temperatures of the boards or t

5、he components increase towards the activation temperatures of the flux (approx. 150 c) they may start to form halides or salts that can carry electricity and cause shorts. they may be cleaned using chemicals or water saponifiers.water solubles or oas must be cleaned since the acids will eat away at

6、the joints.paste - fluxeshttp:/ na. k.c.p.snb. mn數值越高黏度越高n 製程方式黏度要求點膠-syringe dispensing200-400 kcps網版-mesh screen printing400-600 kcps鋼版鋼版-stencil printing400-1200 kcps錫膏黏度http:/ (最好使用錫膏冷藏設備)。n錫膏不用時一定要蓋好蓋子。n錫膏最好不要停留在鋼板上超過1 小時以上,錫膏不要使用 2 次。n錫膏回溫必須要 2 小時以上。n錫膏不能用任何方式回溫。http:/ (續)n錫膏使用前一定要攪拌, (攪拌時一定要同

7、方向, 3-5分鐘,工具要用圓角,不傷錫粒子) 。n錫膏接拌機之缺點:高速離心會破坯化學成分。n錫膏使用溫度問題: a. 21 印刷不好印 (較不黏), 但好脫模。b. 25印刷好印 (較黏),但脫模不易。http:/ 1 (mil)n雷射切割法為 0.3 (mil)nmark 點到零件中心點之座標偏移量n不只鋼板有偏移量, pc板偏移量:0.5miln機器本身 (指 vision) 也有偏移量:0.5miln所以通常要看鋼板準確性 (誤差範圍)。http:/ (節省時間,降低擦拭紙用量)n得到良好的銲點n減少錫球的產生鋼版製作 http:/ - chemical etchn雷射切割 - la

8、ser cutn電鑄 - electroformed鋼版製作方式http:/ - chemical etchn照相製版n酸液蝕刻n價格便宜n印刷後脫模不易http:/ file資料,與pcb有一致性n精度比化學蝕刻高n表面粗糙n價格比化學蝕貴n印刷後容易脫模鋼版製作 - laser cuthttp:/ - electroformedhttp:/ aperture profileshttp:/ etch stencil aperture (250x)鋼版製作http:/ cut stencil aperture (250x) 鋼版製作http:/ stencil aperture (250x)鋼

9、版製作http:/ 20%通常以銲墊的寬度 (w) 為基準一般設計原則:wtw : 寬度 t : 厚度aspect ratio (a.r.) = w / t 1.5width = 4-5 particle diametersrecommended sizes :pitchpad width鋼板開孔 鋼板厚度 a.r. 25 15 12 62.0 20 12 9 -10 5 - 61.7 15 10 7 - 8 51.4 12 8 5 - 6 4 - 51.2鋼版製作- aperture designhttp:/ bga aperturesn針對bgas 與 chip scale package

10、s (csps) 而言, 鋼板開孔的長度與寬是一樣的。通常 pitch 20 mil 以下開模用正方形開孔。pitch 20 mil 以上開模用圓形開孔。the recommended ratio of the surface area of the opening to the surface area of the walls of the stencil apertures should be greater than 0.66http:/ = l x wawalls = 2 (l x t) + 2 (w x t)lwtpitchpad鋼板開孔(dia) 鋼板厚度 area ratiob

11、ga60 32 30 6 - 8 1.25 - 0.9450 25 22 6 - 8 0.92 - 0.6920 12 10 5 - 6 0.50 - 0.42dtaopening = p p d2/4awall = p p d t= d / (4 t)鋼版製作- bga apertureshttp:/ it is beneficial to use double sided reflow instead of using a solder wave for the secondary side of the board through hole printing can be utilize

12、d. the formula used to determine the aperture size is: the volume of the through hole minus the volume of the pin, all multiplied by 2 (due to the metal to flux volume ratio of the solder paste).once the necessary volume of paste is determined the aperture size must be calculated from the required s

13、tencil thickness.volume of paste = (vthrough hole - vpin) x 2 volume of a cylinder = p p r2 hnote that the height or length of the pin is the amount of the pin contained within the through hole (thickness of the board).rh鋼版製作- through holehttp:/ the necessary volume of paste is determined the apertu

14、re size must be calculated from the required stencil thickness. if the aperture size is determined to be too large a percentage of hole fill may need to be applied.fillet solder volumethis is the volume of solder that forms the fillet in and around the lead. it does not include the solder in the pla

15、ted thru-hole.the area of the fillet is:a = 0.215 x (r x r)the location of the center of gravity for a fillet is:x = 0.2234 x r + awhere “a” is the radius of the contact.the path of the fillet is a circle, therefore the:length of path = 2p p xtherefore the volume of the fillet is:v (fillet) = 0.215

16、(r x r) x 2p p (0.2234 x r + a)volume of paste = (vthrough hole - vpin + vfillet) x 2鋼版製作- through holehttp:/ percentage of hole fill required must be determined once the maximum allowable aperture size and stencil thickness is found.from the previous slide, once the volume of paste is found, subtra

17、ct the volume of paste by the volume of the aperture. this is the additional volume required.vpaste = (vthrough hole - vpin + vfillet) x 2vaperture = l x w x stencil thickness= 3.14 x r2 x stencil thicknessadditional volume = vpaste - vaperture.hole fill %= additional volume / vpastefor example:hole

18、 diameter = 0.027”board thickness = 0.062”pin diameter = 0.018”stencil thickness = 0.006”aperture size = 0.055”x 0.055”vpaste = 39443 mils3vaperture =18150 mils3vadditional = 21293 mils3% fill = 60%鋼版製作- through holehttp:/ patternxxyaaperture designxxyathe “home plate” design for chips was created t

19、o assist in reducing the effect of solderballing when using no-clean solder pastes, when too much paste is applied and too much solder is trapped beneath the part.dfxstencils passive devicesa = l(max) / 2.4 (approx)y = w(nom) + 0.008”x = (l(max) a) *0.075d = 0.8 * xf = y - 0.002”where:l(max) and (mi

20、n) are the component lengthw(nominal) is the average component widthwlthx / 4f / 3http:/ 具有良好硬度且板緣沒有毛邊npcb 低變型量(warpage)nfiducials 位於pcb的對角n鋼板孔模應該少於銲墊 20%,否則將容易造成短路,橋接與錫球board / pad 設計http:/ n綠漆高度應該低於銲墊(pad)高度。 n綠漆不可印到pad 。board / solder mask(綠漆)http:/ / shifted solder mask綠漆印刷偏移綠漆印刷偏移 20 mil pitch

21、零件零件. (12 mil pad)misaligned solder mask. solder mask is on top of pad. 綠漆蓋到綠漆蓋到padpad12 mil pitch device. (8 mil pad)26board / mask problemshttp:/ / mask problemspaste 溢出溢出solder mask over padssilkscreen, and labels can prop up the stencil and prevents good gasketing which causes bleed out.綠漆太厚標籤貼紙

22、http:/ copper with coating (osp or silver)flattersilver / alpha level: enhanced solderability.ntin / lead haslflatness is board vendor dependent.minimize bubbles, shift, waves to provide better gasketing, may effect vision.horizontal diagonal hot air leveling is recommended.maintain 0.0006” variatio

23、n on pads / device.pad metalizationhttp:/ hasl. illustrating some pooling in the direction of leveling.diagonal horizontal hasl. no pooling effect.pad metalizationhttp:/ scan of a 20 mil pitch devicewith copper lands.cyber scan of a 20 mil pitch devicewith hasl lands.30pad metalizationhttp:/ metal a

24、nd polyurethane trailing edgesqueegee typeshttp:/ effect of scooping or scavenging.lasts longer than poly blades.can be fragile.most popular.metalsqueegeehttp:/ vary paste deposit by initiating scooping.less expensive than metal.may be necessary if using a step-down stencil.90 or higher durometer re

25、commended.polyurethanesqueegeehttp:/ blade - scoopinghttp:/ modelssnapoff vs. on-contact printingproper board supportsqueegee pressure, downstop and speed.stencil wiping, (vacuum wipe)post print verificationenvironmental controlprinting parametershttp:/ modelsuse local fiducials on both the stencil

26、and the board. global fiducials can be disturbed from board routing and decrease accuracy.choose fiducial across the diagonal of the board. this enables the printer to make the best angular correction.apertures and pads may be used but accuracy will decrease as the aperture fills with paste.uniquene

27、ssvision modelshttp:/ modelsfiducial teachingpad / aperture teachingleave about 0.25” or more space around the fiducial for the 1st box. this is the actual model that the machine looks for during alignment. this must be unique in the field of view.make the second box (cross hair box) fit tightly aro

28、und the fiducial chosen.if there are no fiducials on the stencil or board, then pads and apertures can be taught as vision models. these must be taught uniquely. plcc end pads or soic end pads are the best to choose. to make these unique the user can box 2 pads in then move the box so that is only h

29、as the end pad contained in it then the user can choose the centroid of the pad (do not box in any traces that exist) in the second step.http:/ vision offsets0.002”0.001”+x- y-x+yx offset = + 0.001”y offset = - 0.002”0.002”0.001”+x+y-x-yx offset = + 0.001”y offset = + 0.002”print offset corrections

30、(ultraprint 3000)the print offsets can be viewed 2 different ways: by looking directly at the board and by looking at the screen on the machine (typically during view errors). when looking at the monitor on the machine it is important to remember that the y axis is upside down. see the 2 illustratio

31、ns below. remember you are moving the board, not the print.looking directly at the printed boardlooking at the monitor during view errorshttp:/ vs. on-contact printingon contact printing combined with a slow snapoff or slow board release will yield the best results, but adds to overall cycle time.co

32、ntact printing reduces the effect of stencil stretch.snapoff printing may be necessary when printing at high speeds, when slow snapoff cannot be used, and when cycle time is critical.snapoff printing may yield varying paste heights when getting closer to edges.snapoff vs. on contacthttp:/ separation

33、slow snapoff issues the separation of the paste depends upon the aspect ratio of the stencil design and the viscosity or rheology of the paste. if the speed of the separation is too fast for the given paste the print deposit will appear to have peaks or dog-ears. this is because the paste is stickin

34、g to the corners of the apertures and the paste is being pulled up. if the speed is too slow, it is possible that the induced vibration can cause slumping of the paste. this can be seen when the paste can be printed fast and the slow snapoff speed is set too low. the user should also make sure that

35、the separation distance is adequate to completely remove the board from the stencil. when warped boards are used vacuum may be needed to hold the board flat. depending upon the routing or via holes in the board the vacuum may pull the stencil with the board over some distance. this makes it necessar

36、y to increase the distance.http:/ support optionsvacuum hold down.side snugging of board.dedicated fixturingproper support and hold down of board is a must for accurate printing. this prevents the board from moving and bowing while printing.above board clamps, vacuum, side snugging may give printing

37、 issuesboard supporthttp:/ force, downstop and speedsqueegee force is the amount of force applied to the stencil and board from the squeegee blades.squeegee downstop is the amount of overtravel that the squeegee travels into the stencil to allow the squeegee to adapt to variations in the board durin

38、g printing.force, downstop & speedhttp:/ and forcedownstop = 2mmforce = 10if too much force is applied the squeegee will not float and the blade will bend too much causing the force to change throughout the stroke, possibly causing bridging.proheaddownstop = 2mmforce = 0when downstop is applied

39、the prohead moves down the amount of the downstop. this allows the downstop block to float.proheaddownstop = 2mmforce = 5if the correct force is applied the squeegee will float over the stencil and maintain the correct blade angle and force throughout the whole stroke.proheaddownstop = 0force = 0pro

40、headproheadproheadap, up2000 and up3000 pro-headhttp:/ squeegee parameters(note: these numbers are paste dependent)squeegee parameterstrailing edgetrailing edge metal blades poly bladesforce1 - 1.5 lb. 1 - 2 lb. (0.45 0.68 kg) (0.45 0.91 kg)per linear inchper linear inchof squeegeeof squeegeedownsto

41、p0.070” - 0.085”0.055” - 0.065”(1.78 2.15 mm)(1.4 1.65 mm)speed0.5 to 6.0 in/sec1 to 6 in/sec(12.7 to 152 mm/sec)(25.4 to 152 mm/sec)http:/ wiping / vacuum wipingpaste, stencil and board dependent.good stencil to board gasketing will reduce the frequency of wiping necessary.snap-off printing and lar

42、ge apertures will increase the wiping frequency.if aperture clogging and opens are an issue, use vacuum wiping option to remove paste from apertures.wiper parametershttp:/ wiper frequencyprint 2-3 boardsjog vision system out to inspect around and inside stencil apertures.repeat this until you see ex

43、cessive squeeze out which will result in bridging.subtract 1 - 2 prints from the determined number of prints for your wipe frequency.wiper parametershttp:/ wipe cyclestart with default settings for a single dry wipe.use vision system to verify cleanliness of stencil.if stencil is not clean, use of s

44、olvent or additional wipes may be necessary.use vacuum wipe if unclogging apertures is necessary.note: excessive delays between prints may require more aggressive wipes (i.e. solvent and vacuum)wiper parametershttp:/ print verificationuse to keep process in control.choose critical components to insp

45、ect.study trends to improve your process.determine when stencil cleaning is necessary.post print - 2d & 3dhttp:/ devices (2d & 3d)correct: the teach box should be sized such that it exactly fits around the outer edge of the pad, but does not include the traces of the pads. use the dotted lin

46、es as guides.incorrect: if the teach box is made too large the contrast of the device may be set too low and the paste may not be seen correctly or it will show up very light (low percentage coverage).incorrect: if the teach box is made too small the contrast of the device may be set too high. this

47、could cause the device to fail during pre-inspection or show abnormally high coverage.http:/ environmentadequate environmental conditions are a necessity for good printing.ntemperature: maintain the operating condition of the paste, typically between 70 and 76 f (21 25c).operating outside of these s

48、ettings may cause slumping or excessive drying out of the paste.nhumidity: maintain approx. 50%less than 50% will result in the paste drying out (the volatiles will evaporate).greater than 50% will cause the paste to absorb moisture in the air and result in outgassing, resulting in voids and solder

49、balls. printing environmenthttp:/ environment (cont.) air flow: keep air flow in the print area to a minimum. increased air flow will drive off the volatiles in the paste resulting in poor printing and reflow.cleanliness: keep the printer clean. solder paste buildup in the worknest or other moving a

50、reas will decrease the effectiveness of the board support and the repeatability of the machine.printing environmenthttp:/ blade test0.00500.001000.001500.002000.002500.003000.003500.004000.004500.005000.0004080120160160 pin qfp (leads)paste volume (cu-mils)poly bladesmetal bladessqueegee blade effec

51、tshttp:/ systemnprogrammable squeegee head.nautomatic paste dispenser.nautomatic stencil wiper with vacuum.nease and repeatability of setup.nrepeatable, stable board support.npost print verification and spc data collection.nautomation and reduction of process variability.nenvironmental conditions (t

52、emp., humidity, etc).printer overviewhttp:/ systemlook up / look downngives machine ability to fine align board to stencil.high speed to enhance cycle time.high degree of repeatability +/- 0.0005”.printer - vision systemhttp:/ squeegee headfully software controlled.full closed loop systemnprecisely

53、controlled pressure.nability to adapt to board variations.repeatable from board to board.printer - squeegee headhttp:/ automatic paste dispenserfully software controlled.reduces printer variability.reduces solder paste waste.helps to maintain a consistent amount of paste on the stencil at all times.

54、paste dispenserhttp:/ automatic wiperfully software controlledvacuum capable for clogged aperturessolvent and dry wipe capablepaste / stencil and board dependentnincreased ratio between stencil aperture and pad will reduce gasketing and increase need for cleaning.ncan use vision system to determine

55、wipe frequency.automatic wiperhttp:/ of a dirty stencilnote: solder paste residue bridging the gaps between the apertures and apertures clogged with paste.automatic wiperhttp:/ after a single wipe with vacuum.automatic wiperhttp:/ and repeatability of setup and board teaching.software ability to tea

56、ch a new pcb in a step by step procedure.quick and easy printing parameter setup.nsqueegee parameters, vision models and offsets, post print information, etc.can all be recalled easily by loading a file.board teach / setuphttp:/ supportvacuum hold down.side snugging of board.proper support and hold

57、down of board is a must for accurate printing. this prevents the board from moving while printing.board supporthttp:/ print verificationto ensure a quality print post print sensing can catch defects such as opens, excessive and insufficient paste heights.keep process in control.bgas may require 100%

58、 inspection.post print - 2d / 3dhttp:/ data collectionkeep track of your process by knowing:nhow well the boards were fabricated.nhow well the vision system is recognizing the board.nif the stencil needs cleaning.n2d and 3d trends.spc datahttp:/ and reduction of process variabilitykeep your process

59、repeatable by:ncritical setup parameters may be recalled easily from saved board profiles.nautomatic placement of board support.nautomatic paste dispense.nautomatic stencil changeover.nautomatic stencil wiping.variability reductionhttp:/ modelsnsnapoff vs. on-contact printingnsqueegee pressure, down

60、stop and speed.nstencil wiping, (vacuum wipe)npost print verificationnenvironmental controlprinting parametershttp:/ modelsuse local fiducials on both the stencil and the board. global fiducials can be disturbed from board routing and decrease accuracy.choose fiducial across the diagonal of the board. this enables th