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Ask and Answer -- November 13, 2002
December 31, 1969 |Estimated reading time: 2 minutes
Ask and Answer is a forum where SMT professionals with questions can meet up with those with the answers. If you know the answers to any of the questions below, or have questions of your own, please e-mail them to christinef@pennwell.com, referencing the reference number below each question, and we will publish them in a future column. Please include your name and the location you are writing from. SMT Online reserves the right to edit responses. Look for Ask and Answer every Wednesday.
QUESTIONSOrlando Pontes from Brazil writes: We intend to reballing BGAs using the reballing fixtures method so we have some questions regarding the stencil design used for paste printing and balls position:
- What (is) the appropriate thickness for the stencil to be used for paste printing to solder the new balls on (the) BGA.
- What (is) the appropriate design for the stencil (aperture size and thickness) to be used for position the balls (Ball Size: 20mils, Ball composition,63SN37PB) on the BGA.
Thank you. Reference Number: 111301
George Lucas writes: Are surface mount metal film resistors and diodes susceptible to cracking like surface mount multilayer ceramic capacitors and what would be the expected failure modes for a cracked surface mount metal film resistor when exposed to a high humidity environment? Reference Number: 111302
Cheryl Johnson writes: I am getting ready to do my first flip chip on flex design and need to know if there is any good reference material I can use to find out the following (or if someone can just tell me the rules of thumb!): 1) "Balls" are ~3 mils as close as 7 mil center to center. 2) What size pad (1:1?) do I put on my copper layer? 3) Soldermask vs. coverlay -- and what kind of openings for each? 4) Can traces be 3-4 mils as well? 5) What kind of notes would be different from standard flex? 6) Would anyone have a sample fab/gerber file set (even in pdf) that they could show me? Thank you. Reference Number: 111303
John Noel Villordon of China writes: I am working in an EMS company that manufactures electronic products and assemblies using surface mount technology. I would like to know how long a time can a standard SMA PCB/PCBA (thickness equal to 1 mm) be exposed in ambient/normal manufacturing temperature (21 to 24 degrees C) & humidity (40 to 60 RH) such that the accumulated moisture is still negligible? What is the moisture sensitivity JEDEC level of a standard double-sided SMA PCB? Thank you. Reference Number: 111304