IPC-7525-Stencil Design Guidelines

STENCIL DESIGN GUIDELINES a ABSTR In mid generate Guideli 2000. IPC 752 question 1. Ap boa 2. Ste 3. Ste Hy 4. Ste 5. Glu 6. Mix des 7. No 8. Pla 9. Cer 10. uBG 11. Mix 12. Per val BACKG SMT Pr issues w assembl assembl like som process product establis stencil approac Introduc The goa IPC doc The doc be impo design. influenc possible followin 1. Ter 2. Ref 3. Ste 4. Ste e e e C s t a : r r s t t o h e l t y e William E. Colem Photo Stencil Colorado Springs C ACT -1998 an IPC subcommittee was formed to a document, IPC 7525, for Stencil Design nes. This document was released in June of This talk will review some of the key issues of 5. In particular, some of the commonly asked s about Stencil Design that will be reviewed are: erture size: Length & Width / reduction from rd pad. ncil thickness ncil technology to use: Chem-Etch, Laser-Cut, brid, Electroformed p / Relief stencil design e stencil aperture design ed technology: Through-Hole / SMT stencil ign -Clean aperture design for Chip components stic BGA stencil design amic BGA stencil design A / CSP stencil design ed technology: SMT / Flip Chip stencil design. cent paste release compared to the theoretical ue of solder brick LxWxT ROUND ocess Engineers often face similar stencil design hen either (a) they are new to the SMT print / y arena or (b) they have a new SMT print / y requirement. The new Process Engineer would e basic stencil design guidelines. The seasoned engineer, who is busy dealing with high volume ion issues, would prefer to draw on an hed experience base in the form of general design guidelines as a starting point. This h often saves man-weeks in New Product tion. l of IPC subcommittee 5-21e is to provide an ument for Stencil Design Guidelines (IPC 7525). ument is intended as a guideline only. It would ssible to establish a firm standard for stencil There are just too many other variables that e stencil design to make a standard design . The document includes sections on the 5. St 6. St 7. St STEN A com design squeeg cycle, then re stencil filled attache solder inner factors 1. Th 2. Th 3. Th The A The ge paste r the A dimen Length Ratio i separa compe stick t pad is wall, t or bett Stenci percen directl stencil inside Cut st release given with m g: ms and Definitions erence Documents ncil Design ncil Fabrication higher main le approa stencils achieve n O ncil Mounting ncil Cleaning ncil End of Life IL APERTURE DESIGN mon question about stencil design is the aperture and how it effects print performance. As the ee blade travels across the stencil during the print older paste fills the stencil apertures. The paste leases to the pads on the board during the board / separation cycle. Ideally, all of the paste that he aperture releases from the aperture walls and s to the pads on the board forming a complete brick. The ability of the paste to release from the perture walls depends primarily on three major e Area Ratio / Aspect Ratio for stencil design e aperture side wall geometry e aperture wall smoothness ea Ratio / Aspect Ratio are defined in Figure 1. nerally accepted design guideline for acceptable elease is >1.5 for the Aspect Ratio and >.66 for ea Ratio. The Aspect Ratio is really a one- ional simplification of the Area Ratio. When the (L) is much larger than the Width (W) the Area s the same as the Aspect Ratio. When the stencil es from the board, paste release encounters a ing process: will it transfer to the Pad or will it the side aperture walls? When the area of the greater than 2/3 of the area of the inside aperture e paste has a good probability of achieving 85% r paste release. technology also plays a major role in the age of solder paste release. Items (2) and (3) are related to stencil technology. A Laser-Cut that is electropolished definitely has smoother aperture walls than a non-electropolished Laser- ncil. It stands to reason that the former will a higher percentage of paste than the latter at a Area Ratio. Likewise an electroformed stencil irror type aperture wall finish, will release even a percentage of paste at the same Area Ratio. The sson to be learned is that for Aspect Ratios that ch 1.5 and Area Ratios that approach .66, some technologies are better suited than others to higher percentages of paste release. Table 1 considers the Aspect Ratio / Area Ratio of some practical examples of aperture design for typical SMD’s. A 20 m mil thi a sten produc perform apertur this is high t should 1. In (W 2. D (T 3. Se ap Flash Typica board pad de defined circula falsely than 1 could b print u the len (two-d paste r difficu recomm smalle rule, m Howev of cop interfe to 13 Area R should mirror Intel h slightl Overal shape apertur Apert As a g reduce apertur origina apertur proces instanc possibility of stencil aperture to board pad misalignment. This reduces the chance for solder paste to be printed off d, which may lead to solder balls or solder r c b t p s i r G 2 n a e a r i e o s s c i c M l o i i i e o c g il pitch QFP with a 10 x 50 mil aperture in a 5 ck stencil has an Aspect Ratio of 2.0. Employing cil technology with smooth aperture walls will e excellent paste release and consistent print ance. A 16 mil pitch QFP with a 7 x 50 mil e in a 5 mil thick stencil has an aspect ratio of 1.4; a very difficult paste release situation; even for echnology stencils. One or all of three options be considered for this situation: crease the aperture width (increasing the width ) to 8 mil increases the Aspect Ratio to 1.6) ecrease the thickness (decreasing the foil thickness ) to 4.4 mil increases the Aspect Ratio to 1.6) lect a stencil technology with very smooth erture walls. Memory uBGA’s are becoming very popular. lly these devices have a 12 mil circular pad on the with a 15 mil solder mask opening. The preferred sign is copper defined rather than solder mask . Consider example 5 which shows an 11 mil r aperture. The Area Ratio is 2.2. One could assume that since the Area Ratio is much greater .5 that paste release is not a problem. Nothing e further from the truth. Anyone who has tried to BGA patterns can quickly affirm this. Anytime gth is less than 5 times the width; the Area Ratio imensional model) needs to be used to predict elease. In this case the Area Ratio is .55; a very lt paste release situation. Normally it is ended to make the stencil aperture slightly r than the board pad. Example 5 abided by this aking the stencil aperture 11 mil for a 12 mil pad. er uBGA are an exception, especially in the case per defined pads. There is no solder mask rence with paste if the stencil aperture is increased mil. This is shown in example 6. Note that the atio is now .65. Even at a .65 Area Ratio one still select a stencil technology that provides smooth inner aperture walls. Both Tessera and ave recommended square stencil apertures with y rounded corners for uBGA stencil printing. l feedback from customers confirms that this aperture provides better paste release than circular es. ure Size versus Board Pad Size eneral design guide, the aperture size should be d compared to the board pad size. The stencil e is commonly modified with respect to the l pad design. Reductions in the area or changes in e shape are often desirable to enhance the ses of printing, reflow, or stencil cleaning. For e, reducing the aperture size will decrease the the pa bridgin apertu are cir size of on the have s Pitch-B Throug BGA’s better second solder issues the th annula increas hole. Table Step a There in a ste 1. St ex fo th 2. R ste sp ga te po Pr te SM /S so Th H m Th m ste pr sm Th 3. St w ex ste ste ed g. Having a minimum radius corner for all es can promote stencil cleaning. However, there umstances where it is desirable to increase the the stencil aperture opening larger than the pad oard. Three examples of where it is desirable to encil apertures larger than board pads are Fine- GA’s, Ceramic BGA’s and Intrusive Reflow h–Hole apertures. In the first case for Fine-Pitch overprint of the board pad is desirable to get aste release by increasing the Area Ratio. In the case for CBGA’s it is desirable to increase the paste volume to minimize any non-coplanarity ince the BGA balls do not melt during reflow. In rd case for Intrusive Reflow overprinting the ring around a Through-Hole is desirable to e the amount of solder available for the pin-in- eneral aperture design guidelines are shown in . d Relief Step Stencil design re a number of cases where steps may be required ncil. Some of these cases are listed below: p-down area for Fine Pitch components. An mple of this case would be an 8 mil thick stencil all components except for 20 mil pitch where a ckness of 6 mils is required. lief step on the board side of the stencil. Relief ps are desirable when there is a protrusion or high t on the board which prevents the stencil from keting during printing. Examples are: barcodes, t via’s, and additive trace lines. Relief step kets are also used for two print stencils. Two nt stencils are used mainly with mixed hnology requirements: either Through- Hole / T or SMT / Flip Chip. In the Through-Hole T case the first stencil prints all of the SMT der paste with a normal thickness stencil (6 mils). e second stencil prints paste for all the Through- le components. This stencil is normally 15 to 25 ls thick to provide sufficient solder paste for the rough-Holes. There is a relief step (typically 10 l deep) on the board side of this second print ncil at all locations where SMT solder paste was nted during the first print. This prevents earing of the SMT solder paste during the rough-Hole printing. p-Up stencils. An example of a Step-Up stencil uld be a stencil that is 6 mil thick in all locations ept in the area of a ceramic BGA where the ncil is 8 mil thick. Another example would be a ncil that is 6 mil thick except in the area of an e Through-Hole connector where the thickness is 10 mil. It is desirable to have the width of the 6 mil thick area at least as wide as the squeegee blade. Mixed Technology Surface-mount/Through-hole (Intrus It is de devices (1) pri (2) pla (3) ref The ob intrusiv volume solder f Solder A simp paste re the Ann possibl betwee the pin paste v stencil paste: (1) No (2) Ste (3) Tw Specia Chip C Several occurre reducin compon Figure MELF For M apertur for the with co Glue A The glu apertur pads. It pad hei STENC The fab or subt electrof subtrac to crea exampl Chemical Etch Chemically etched stencils are produced using photo- imageable resist laminated on both sides of metal foils cut to s i e c h w e d w s t e f g e e s n a i s a b a L d a e t b s ive Reflow) sirable to have a process where SMT and THT can both be: nted with printed solder paste ced on or in the board lowed together. jective of stencil printing of solder paste for the e reflow process is to provide enough solder after reflow to fill the hole and create acceptable illets around the pins. Paste Volume le equation below describes the volume of solder quired as shown in Table 3. It is desirable to keep ular ring pad around the through-hole as small as e. It is also desirable to keep the clearance n the pin and the through-hole and the length of as small as possible. By doing this, less solder olume will be required. Following are three designs used to deliver the through-hole solder n-step stencil, Figure 2 p stencil, Figure 3 o print stencil, Figure 4 l Aperture Shapes omponents - Resistors and Capacitors aperture geometries are effective in reducing the nce of solder balls. All these designs are aimed at g excess solder paste trapped under the chip ent. The most popular designs are shown in 5 and 6. , MINI-MELF Components ELF and Mini-MELF components, "C" shaped es are suggested. (See Figure 7) Stencil openings se components should be considered to match mponent terminals. perture Chip Component e stencil is typically 6 – 8 mil thick. The glue e is placed in the center of the component solder is 1/3 the spacing between pads and 110% of the ght. (See Figure 8) IL FABRICATION TECHNOLIGIES rication process for stencils may involve additive ractive methods. In additive processes such as orming, metal is added to form apertures. In tive processes, metal is removed from stencil foils te apertures. Laser-cut and chemical etch are es of subtractive processes. held in is used resist. reduced dimens describ as the foil. T metal etched apertur strippe Laser C Laser c by soft etched are cut inheren specifie the squ Electro Electro utilizin process Thickn thickne resist a where pillars electrop thickne plating and the Hybrid Where present a com stencils combin CONC One of stencil aspect/ shrink, shrink. thus sm translat more s should provide pecific frame sizes. A double-sided phototool, precise alignment usually with registration pins, to expose the stencil aperture image onto the Aperture images exposed on the resists are in size compared with the desired aperture ons, accounted by an etch factor. The etch factor s the amount of lateral etching that takes place hemical etches through the thickness of metal e exposed resist is then developed, leaving bare here apertures are desired. The metal foil is from both sides in a liquid chemical, creating s as specified. The remaining resist is then away and a stencil foil is produced. ut ut stencils are produced from Gerber® data run are of the laser equipment. Unlike chemically tencils, no phototool is required. Since stencils from one side only, tapered aperture wall is an part of laser cut stencils. Unless otherwise d, apertures are larger on the board side than on egee side. form orming is an additive stencil fabrication method photo-imageable resist and an electroplating s. Photo-resist is placed on a metal mandrel. ss of the resist is greater than the final stencil s desired. The apertures are imaged onto the d the resist is developed, leaving resist pillars pertures are desired. The mandrel with resist s placed in a nickel plating tank where nickel is lated onto the mandrel. When the desired foil s is reached, the mandrel is removed from the tank. Lastly, the photo resist pillars are stripped nickel stencil foil is separated from the mandrel. mixture of standard and fine pitch assemblies is on a board, the stencil fabrication process may be ination of laser cut and chemical etch. The produced are referred to as laser-chem tion or hybrid stencils. USION the most important design considerations for esign (aperture size and stencil thickness) is the rea ratio. As the size of electronic packages the center to center spacing of the I/O leads also This, in turn, means smaller I/O board pads and aller stencil apertures. Smaller stencil apertures s into smaller aspect/area ratios, which puts ringent demands on stencil performance. Care e exercised in selecting a stencil technology that high performance. It is essential to consider the aspect/area ratio and keep these ratios above 1.5 and .66 respectively when designing the stencil apertures. Overprinting (when the stencil aperture is larger than the board pad) is very useful for several applications including intrusive reflow, fine-pitch BGA’s, and ceramic BGA’s. Special aperture shapes are useful for eliminating free micro solder balls in no-clean paste applications. ACKNOWLEDGEMENTS The author would like to acknowledge the contributions of all who server on the IPC subcommittee 5-21e. Without their work and dedication the release of IPC7525 would not have been possible. Special recognition goes to Kathy Jenczewski, Charlie Davis, Kermit Aguayo, Kantesh Doss and Nick Mescia. A special thanks to Jane Koh of IPC for outstanding support of this effort. A T E 5 uBGA 30 PITCH 11 SQUARE 5 (THICK) 2.2 .55 +++ .6 .65 ++ Figure 1: Stencil Aperture 6 uBGA 30 PITCH 13 SQUARE 5 (THICK) 2 + Indicates degree of difficulty Aspect Ratio = Stencil of Thickness Aperture ofWidth = T W rea Ratio = WallsAperture of Area Aperture of Area = T W) (L 2 W L ×+× × able 1 Examples of Aspect/Area Ratios for Various Surface Mount Devices ASPECT AREA PASTE XAMPLE APERTURE DESIGN RATIO RATIO RELEASE 1 QFP 20 PITCH 10 X 50 X 5 (THICK) 2.0 .83 + 2 QFP 16 PITCH 7 X 50 X 5 (THICK) 1.4 .61 +++ 3 BGA 50 PITCH 25 CIRCLE 6 (THICK) 4.2 1.04 + 4 BGA 40 PITCH 15 CIRCLE 5 (THICK) 3.0 0.75 ++ Table 2 GENERAL APERTURE DESIGN GUIDELINES FOR SMT PART TYPE PITCH PAD APERETURE APERTURE STENCIL ASPECT AREA FOOTPRINT WIDTH LENGTH THICKNESS RATIO RATIO NGE PLCC .07-1.17 QFP .71-.83 QFP .69-.83 QFP .68-.86 QFP .65-.86 0402 N/A 20X30 18 22 5-6 N/A .83-.99 0201 BGA uBGA uBGA uBGA NOTE N/A 10X20 8 16 3-4 N/A .66-.89 50 32 CIR 30 CIR 30 CIR 6-8 N/A .93-1.25 40 15 CIR 14 SQ 14 SQ 4.5-5.25 N/A .67-.78 30 12 14SQ 14SQ 4.5-5.25 N/A .67-.78 20 12 CIR 11 SQ 11 SQ 3-4 N/A .69-.92 S 1 It is assumed that the uBGA pads are not solder mask defined 2 The uBGA apertures are square apertures with radiuses corners 3 mil for the 14 mil aperture and 2.5 mil for the 11 mil aperture 3 All dimensions are in mils and rounded from the true metric, ie .65mm is 25 mils .5 mm is 20 mils etc. Ratios are dimensionless 4 N/A implies that the Area Ratio should be considered only. RANGE RANGE RA 50 25 23 100 8-10 2.3-2.9 1 25 14 12 60 6-7 1.7-2.0 20 12 10 50 5-6 1.7-2.0 16 10 8 50 4-5 1.6-2.0 12 8 6 40 3-4 1.5-2.0 Table 3 Paste-In-Hole Equation ( ) ( ){ } HPBTpHBS1 OO S V V F F A - AT ) WL(T V −+++= ×= Where : V is volume of solder paste required VP is the solder volume left on the top and/or bottom board pad S is the solder paste shrink factor AH is the cross sectional area of the through-hole Ap is the cross sectional area of the through-hole pin TB is the thickness of the board FT + FB is the total fillet volume required TS is the thickness of the stencil LO is the length of the overprint aperture Figure 2: Overprint without Step Figure 3: Overprint with Step (Squeegee Side) Figure 4: Two Print Through-hole Stencils Figure 5: Home Plate A Figure 8: Glue Stencil Aperture Design perture Design Figure 6: Bow Tie Aperture Design Figure7: Aperture Design for MELF Components MAIN MENU PREVIOUS MENU --------------------------------- Search CD-ROM Search Results Print