Hi Lothar,
I wasn't aware it was possible to get solder mask between the pads on a 0.4mm pitch device! Solder bridging is definitely the biggest problem with soldering 0.4 mm pitch devices, and having solder mask between the pads would certainly help. However, using world class PCB suppliers who can keep a 100µm solder mask web aligned within ±50µm, still only leaves a 0.2mm (0.008") surface mount pad width. More typical is ±65µm alignment and a 125µm solder mask web, leaving a 0.15mm (0.006") surface mount pad width.
Assuming a pad of only 0.2mm, and since the board shop is allowed a tolerance in the pattern dimensions, they will typically 'over etch' the pattern to prevent copper shorts. And, since the pad cross-section has a taper of typically 30µm, the top surface of the pad might only be 0.15mm (0.006") or less. If the pads are only 0.15mm wide, it is difficult to align the leads of the component perfectly with the pad. This results in some or all of the leads being slightly off the pad. Assuming IPC class two, up to 50% of the lead may be off the pad, but having a larger pad allows the lead to shift a greater distance without exceeding the 50% rule. (This problem is particularly vexing for process engineers when the component lead is larger than the PCB pad; as would be the case of a 0.15mm top pad.)
0.4mm pitch devices requires 'riding the edge' of process capability. There is a delicate balance of what the board shop will do, what the pick and place equipment can do, what stencil design can gain, what solder printing can do, and what the quality the component manufacture will supply.
In other words, the request of your assembler is reasonable, even if their reasoning is silly. The 0.125mm pad gap makes me nervous, but in reality, the incoming bare boards will actually have an effective gap that is greater than 0.125mm.
Fortunately for you, you are dealing with a supplier. Our design department has been duly notified that if they ever design another board with a 0.4mm pitch devices, an angry mob of process engineers, technicians, production operators and supervisors will come with pitch forks and torches to kill the monster. (We demand BGA's packages are used instead of 0.4mm pitch QFP's)
Regards,
Ryan
-----Original Message-----
From: Leadfree [mailto:[log in to unmask]] On Behalf Of Lothar Thole
Sent: Monday, May 21, 2007 1:47 AM
To: [log in to unmask]
Subject: [LF] No soldermask between SQFP leads ok?
Thanks again to the responses to my question on asymmetric microBGA
joints.
Another question on land patterns if I may:
Our new leadfree assembler has requested us to modify our current
SQFP Land Patterns by making the pads wider to allow for Side Fillets,
thus improving the reliability of the solder joint. This will result in a
situation where the 0.4mm (0.016in) pitch devices have a Pad-Pad
spacing of 0.125mm (0.005in) with no dividing Soldermask. Our concern
now is the possibility of Solder Bridging. Will the benefits of the side
fillets outweigh the risks of bridging? Thanks in advance.
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