Printed Circuit Board Assembly & PCB Design SMT Electronics Assembly Manufacturing Forum

Printed Circuit Board Assembly & PCB Design Forum

SMT electronics assembly manufacturing forum.


Rich

#15486

PCB Standards | 10 June, 1998

I am looking for the Industry standards on vias. Are they required to be plated with solder or is copper plating enough? What about reliabilty with either one? Also, if tenting the via is required should it be copper plated or solder plated over copper? Second area, are smd devices required to have a nickel barrier under the solder coating for capacitors and thermistors? What standards specify these areas? Third area, what is the standard for allowable ionic contamination? I've heard 18.5 ug/in2. Any help is appreciated! Rich

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Earl Moon

#15487

Re: PCB Standards | 10 June, 1998

| I am looking for the Industry standards on vias. Are they required to be plated with solder or is copper plating enough? What about reliabilty with either one? Also, if tenting the via is required should it be copper plated or solder plated over copper? | Second area, are smd devices required to have a nickel barrier under the solder coating for capacitors and thermistors? What standards specify these areas? | Third area, what is the standard for allowable ionic contamination? I've heard 18.5 ug/in2. | Any help is appreciated! | Rich

Rich, First you cannot have tin/lead in a drilled hole, or any other via, without first having copper metallization. To effect acceptable conditions holes must first be drilled (or?), then prepared (by removing resin smear on inner layer conductor interfaces) to ensure an acceptable plating interface. Then, an catalytic electroless copper deposition process is performed on the drilled hole and inner layer conductor (and PCB surface metal) interface by applying sufficient metal to support electroplating. This requires certain things must be met in terms of minimal voiding, etc. Electroplated copper is then deposited over the electroless copper in a thickness not less than .001" (typical but I and many fabricators require 1.2 mils minimum with a minimum ductility (elongation characteristic) of about 20% (IPC and others allow less) and a tensile strength of about 40,000 PSI. All this keeps hole walls together during thermal stress and shock encountered during assembly processes and during operation (as most resin systems found in PCB's expand in the Z axis at a rate of about 50-60ppm/C.. Tin/lead, or some other coating or plating, is then applied (as electroless or electroplated metallizations or organics) over the copper to prevent oxidation thereby assuring solder wetting - under effectively managed process conditions. Industry guidelines or military specifications, as IPC-275, IPC-600 or MIL-P-55110 and its other IPC counterparts, are used to effect design requirements, master drawing callouts, and provide acceptance criteria. Earl Moon

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