Hi mates,
have this SMT production issue :
A) We have a leadless-chip-carrier(LCC) IC package that is classified under the Land-Grid-Array(LGA) package family.
B) The LCC is 4mm x 4mm, and has 28 tin plated pads located beneath the outside perimeter of package body (think of BGA without the solder-bumps beneath the body).
C) The side of the package body perimeter has exposed copper leadless terminals corresponding in width and aignment to each of the 28 (tin-plated) pads beneath the body. The customer insists that this bare copper is its natural condition for the leadless terminal walls.
D) the solder we get has inconsistent solder-fusion onto the exposed portion copper of the leadless terminal walls. customer has classified the gap as the "unsolder" defect.
E) we did analysis as follows :
E.1) there is copper oxidization of the leadless terminal walls. it varies heavier with random frequency on each batch of LCC delivery to us (consignment).
E.2) we did evaluation by manual solder-dip some of the LCC with copper oxidization and found that solder-dip "Tin plated" LCC has excellent solder fusion onto the (now tin coating) leadless terminal walls. the Tin coating negates the copper oxidization layers as Tin assists solder fusion, we therefore get good solder fusion, ie. no "unsolder" gap :P
E.3) If we do get some tin coating "smearing" onto the copper leadless terminal walls at the raw materials stage (still in Tape and Reel format), it is a "bonus" side-effect of the die-slice-cutting during the LCC mfg process. customer is not going nor willing to change any raw materials yet.
E.4) we took a transparency of the : a) Stencil aperture openings b) PCB (Au/Ni) pads sizes and found the two coverage areas are mis-register, when superimposed on each transparency.
E.5) the PCB pads have "up/down/in-out" Pads irregular pad layout; even when the gerber data shows all the Pads are supposedly equal in Pads dimensional lengths and equal dimensional PCB layouts.
E.6) the stencil was generated using customer supplied gerber data of the silk-screen layer. this gerber supplied to us assumes all pads are of equal layout and individual pad sizes/lengths. this assumption is flawed as using a scanner machine we find there are 5 different varient pads dimensions measured, and PCB pad layout is irregular.
E.7) we have calculated the expected deposited paste volume for each individual PCB pad, and had the stencil fab house scan each pad to get accurate dimensions. a new stencil is fab according to the scan data of the actual PCB pads.
E.8) irregular PCB pad layout is found NOT matching the actual LCC component pads layout. some PCB pads is not covering enough land width to accomodate the intended covergae surface area, assuming SMT mounting of the LCC component is accurate with ref to the overall fudical marks.
F) however despite the controlled volume paste printed using , we still see gap between the solder fusion at the contact line to the BCC copper leadless terminal wall. the volume control gives good solder fillet at the contact region nearest to the PCB pads. only problem is now the gap "no fusion of solder" onto the copper walls.
G) we have adjusted the reflow profile to a longer reflow time(60-76sec) and higher peak temperature(212-215) deg-C.
H) recommened specs of paste supplier was violated only for the peak temperature adjustment to (216-218) deg-C to heat up the copper leadless terminal walls and observed that this gives good solder fusion onto the walls. gap unsolder is greatly minimized but we are going for 100% elimination. we tried another fluxing system from same paste maker but cannot get a smooth shinny solder fillet finish.
I) we discussed results with the customer. but they are asking us to try and get the wetting onto the leadless terminals (ie. no gap solder condition) as they have huge surplus stock of the LCC components. they also claim its a off-shelf part number, though we cannot find the LCC part no. in the supplier website database.
J) bottom line of this thread, based on the info given here, can anyone give some suggestions what we might be missing here? any experiences on LGA to share, that can help us out here?
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