Re: DFM Guidelines or solderability?

I have a customer who is transitioning from 100% thru hole boards to designing boards with smt components. We gave them our basic DFM guidelines and they have laid out 2 boards following these guidelines. When I saw the result of their first pass, I thought of two things. 1) we need to review our guidelines and 2) I need some advice from SMTNet. The product is mil-aero building to IPC Class 3 using mil spec components. The assemblies are all laid out for a process flow of 1) reflow topside smt 2) epoxy bottomside smt 3) stuff thruhole 4) wave solder I have 2 questions: 1) They have placed smt resistors as large as 2512's and capacitors up to 1812 on the bottomside. I recall reading about problems with wave soldering large capacitors and having reliability problems. Would this also apply to resistors? What about the 1812 caps? 2) They have also place 1206 resistors between the pins of .300 dips on the secondary side. We had them orient the resistors parallel to the wave but unfortunately the dips are also oriented parallel to the wave (I need to check our DFM doc). With this, they have positioned 6 1206's between the leads of a dip16 component and place 3 dip components in a row with 3 1206s between them. ASCII art to follow. o - reps a dip lead [] - reps a 1206 [] <----- conveyor direction o [] [] o [] o [] [] o [] o o o [] [] o [] repeated 3 times in this direction o [] [] o [] o o o [] [] o [] o [] [] o [] Is anyone building anything like this successfully? Volumes are less than 100/mo. Any input is appreciated. Peter Wong

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Re: Questions #2 Are you having solderability problems with that arrangement? Sounds like problems could be solved at the wave solder. If the customer has already spun pcb then sometimes for low volume it would be easier to correct the problem at wave solder rather than convince customer to respin. Do you have bridging problems (Rotate 90 degrees if pins bridging and use chip wave to soldr 1206's)? Voids (smt)? Excess? Do you have turbulent chip wave? If you could be more specific on the problem I could probably help. I have built such assemblies with 1206,0805...oriented in both directions and pins with .050" centers succesfully. | I have a customer who is transitioning from 100% thru hole boards to designing boards with smt components. | We gave them our basic DFM guidelines and they have laid out 2 boards following these guidelines. When I saw the result of their first pass, I thought of two things. 1) we need to review our guidelines and 2) I need some advice from SMTNet. | The product is mil-aero building to IPC Class 3 using mil spec components. | The assemblies are all laid out for a process flow of | 1) reflow topside smt | 2) epoxy bottomside smt | 3) stuff thruhole | 4) wave solder | I have 2 questions: | 1) They have placed smt resistors as large as 2512's and capacitors up to 1812 on the bottomside. I recall reading about problems with wave soldering large capacitors and having reliability problems. Would this also apply to resistors? What about the 1812 caps? | 2) They have also place 1206 resistors between the pins of .300 dips on the secondary side. We had them orient the resistors parallel to the wave but unfortunately the dips are also oriented parallel to the wave (I need to check our DFM doc). With this, they have positioned 6 1206's between the leads of a dip16 component and place 3 dip components in a row with 3 1206s between them. | ASCII art to follow. | o - reps a dip lead | [] - reps a 1206 | [] | <----- conveyor direction | o [] [] o [] | o [] [] o [] | o o | o [] [] o [] repeated 3 times in this direction | o [] [] o [] | o o | o [] [] o [] | o [] [] o [] | Is anyone building anything like this successfully? Volumes are less than 100/mo. | Any input is appreciated. | Peter Wong

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We haven't built these yet. The customer was in layout at the time and I was looking for any flags from others to determine if I should have them change the layout as I've not seen this before. From your response and others that I've received, it sounds like a manufacturable design. We do have a wave solder machine with forced air convection and a turbulent wave to accomodate this if needed. Thanks for the feedback. Peter

| Re: Questions #2 | Are you having solderability problems with that arrangement? Sounds like problems could be solved at the wave solder. If the customer has already spun pcb then sometimes for low volume it would be easier to correct the problem at wave solder rather than convince customer to respin. Do you have bridging problems (Rotate 90 degrees if pins bridging and use chip wave to soldr 1206's)? Voids (smt)? Excess? Do you have turbulent chip wave? If you could be more specific on the problem I could probably help. I have built such assemblies with 1206,0805...oriented in both directions and pins with .050" centers succesfully.

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