Gary, Thanks for your comments. I have a question about the data of the aperture sizes: Is there a reference where I can get this data or you obtained this data? My question is due to we are looking for some "ideal" aperture size, actually we are testing the aperture size recommended by the stencil vendor. I cross-sectioned some components and I found that there are some components that don't have solder under their terminals. They only have solder at the end of the terminal. Is it acceptable this condition? Regards, Alejandro Becerra -----Original Message----- From: Smith, Gary [mailto:[log in to unmask]] Sent: Thursday, July 13, 2000 9:05 AM To: 'TechNet E-Mail Forum.'; Becerra Alejandro Subject: RE: [TN] Adhesive over pads. Al, excerpts from my research paper (currently STILL in progress). Since I'm only an intern I can't say this info is accurate due to the project not being completed: Adhesive dot profile (size) is also controlled by the internal diameter/standoff height ratios of the selected nozzle. Typically, dot height-to-width ratios range from 1:5 to 1:1.5 (h/w = 0.20 - 0.67) mils, depending on the dispensing system and adhesive grade. The dot size and volume of the adhesive are more critical factors than the dot shape. The dot must have sufficient height after dispensation to be greater than the combined thickness of the pad/land on the PCB and the thickness of the SMT component's metallization (solder contact area). After the attachment of the SMT component to the PCB substrate, a gap must exist between the edge of the adhesive dot and the PCB pads. The illustrations on the following page assist in clarification of these principles. 1 Industry standards have established no minimum gap between the edge of the adhesive dot and the edge of the pad. The essential requirement is that the adhesive does not get on the pad. It can get almost to the edge and may even touch it but it should not get on the pad. In a case of adhesive touching the pads of the substrate, an excessive volume of adhesive was dispensed. The distance between the bottom of the component and the top of the PC board is typically between 0.003" to 0.005" (3-5 mils). Another good rule of thumb is to utilize a dot height that is twice the distance between the bottom of the component and the top of the PC board. This will assure a good bond or wetting of the adhesive between the board's surface and the bottom of the component during placement onto the PC board. Quantification of adhesive dot dispense volume will depend on the method selected. If a stencil process is utilized the amount dispensed is dependent upon the thickness of the stencil and the diameter of the aperture. The total dispense volume per dot is calculated by utilizing the standard geometric formula for the volume of a cylinder: V = P (D/2)2 h · V - total volume of adhesive dispensed per dot · D - diameter of the stencil aperture · h - Thickness (Height) of the stencil NOTE: Since stencil adhesive printing allows for other shapes besides circular ones to be printed the volume is dependent upon the stencil thickness times the area of the aperture opening. Listed in the table below are approximate recommended aperture sizes for different components when printing with a metal, laser-cut stencil of 10-mil thickness using the printing parameters described. Component Size Single Dot Approach (stencil opening diameter in mils) Typical Multiple Dot Pitch (pitch in mils) Multiple Dot Approach ** (stencil opening diameter in mils) 0402 12 - 16 N/A N/A 0603 16 - 20 15 2 X 20 0805 20 - 24 20 2 X 24 1206 40 - 47 24 2 X 32 Mini MELF 40 N/A N/A SOT 23 40 28 2 X 28 1812 50 - 60 40 2 X 55 SO 8 N/A 43 3 X 55 SO 14 N/A 43 3 X 55 ** For the Multiple Dot Approach for chip (leadless) components, position the dots so that between 0.5 and 0.3 of the dot area is under the body of the component. The Multiple Dot Approach is very effective for preventing skewing of chip components, especially MELF (Metal Electrode Leadless Face) types. -----Original Message----- From: Becerra Alejandro [mailto:[log in to unmask]] Sent: Thursday, July 13, 2000 10:25 AM To: [log in to unmask] Subject: [TN] Adhesive over pads. Hi to all, We are testing stencil-printing process for adhesive dispensing in bottom side for 0603 components. We found that there are some adhesive dots that are touching the pads. We found that the maximum distance that the dot penetrates the pad is approximately 0.0025". Is there any standard that defines if it is acceptable to have a small quantity of adhesive over the pads? Is there any thumb rule that indicates the size of the stencil aperture. We have a 0.028" clearance between the pads, and an average dot size of 0.0225". Thanks, Alejandro Becerra Phone (915) 841-8518, Fax (915) 841-8518 [log in to unmask] ############################################################## TechNet Mail List provided as a free service by IPC using LISTSERV 1.8c ############################################################## To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TECHNET <your full name> To unsubscribe: SIGNOFF TECHNET ############################################################## Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information. 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