In his 32 years of industry experience, Mr. Depending on your end customer you may need to provide reliability data. Profile the PCB (with all the components populated) to be sure the BGA solder joints achieve the desired temperature, then cross section the solder joints to make sure the solder joint looks homogenous. There are a few vendors today who offer "transition" solder pastes, that are Sn/Pb alloy based, with a flux medium that can survive higher temperatures. Therefore you need to insure that you Sn/Pb flux system is capable of withstanding the higher temperatures without suffering flux exhaustion. Of course it is going to be higher than a standard tin-lead reflow temperature. This paper also provides the methods to calculate the reflow temperature needed in order to get complete mixing (based on the bump size, alloy, and paste deposit). (IPC/JEDEC, San Jose, 2005) summarized some of the previous work, including reliability studies done by Solectron where they showed an impact in reliability, but in some cases may be acceptable, as long as the correct temperature is achieved in order to get complete mixing. (CMAP, Toronto, 2005) showed the disastrous consequences of not completely reflowing the solder joint and having a Pb-rich region in the solder joint. This typically occurs when a "standard" Sn/Pb reflow process is used, the incomplete mixing will have a significant impact on reliability Hillman, et al. When these are used in a Sn/Pb process there can be a significant impact on reliability.Ī worst case scenario is where the SAC bump is not reflowed or only partially reflowed. This in turn causes the package manufactures to produce predominately Pb-free bumped packages. Manufacturing with a mixed metal system is, unfortunately, becoming more common as high volume manufacturers move to Pb-free. Using a water soluble 63Sn/37Pb paste, what are the optimum profile settings to assure no voids or "IPC acceptable" voids?įor settings I'm looking for times and temperatures for both the soak and above liquidus zones. Note that in the graph above the "ramp rate" is actually measured as 0.75☌/s and is from ambient to peak temperature (not 1.We have an ENIG plated PCB that is entirely leaded except for one BGA which has SAC305 for balls. The reflow process window is becoming very narrow and this attribute (ramp rate) has become as important as time above liquidus and peak temperature. This rate becomes even more important as the solder paste deposit continually decreases in size - as we move to 0201’s and smaller and from 0.5mm pitch BGA’s. Due to this miniaturization, the emergence of a defect known as " graping" has also become fairly well known. This gently evaporates volatiles and helps minimize solder defects such as solder balling, solder beading, and tombstoning. And is more practically used in a ramp-to-spike type profileįrom the view point of the solder paste, a low ramp rate is desired, usually 1-2☌/s. The ramp rate may be better described as the rate (change in temperature over time) from ambient (room temperature) to peak. The focus of max slope is more from a component view point, to avoid thermal shock, usually 3☌/s is recommended as the upper limit The change in temperature between ambient and the first zone then is a minimum of 75☌ (assuming 25☌ as ambient) and so it’s easy to see that the greatest change in temperature (max slope) in most cases is typically found in the first zone The max slope is very often attained in the first zone as the PCB moves from ambient temperature into the oven. In most cases the oven zone setting for the first zone is 100☌ or better. The reason for addressing this subject is that, often, there has been some confusion in regard to the difference between max slope (a category reported on most profiling software) and the ramp rate listed on a data sheet. The data sheet gives general recommendations, for time above liquidus, peak temperature, and ramp rate. Included in the Product Data Sheet, among other things, are parameters which guide the customer in designing an SMT reflow profile. Exactly HOW that is done is critical to your success. Solder paste is made to be reflowed in the SMT process. SiP & Heterogeneous Integration & Assembly (HIA).The Indium Corporation & Macartney Family Foundation.
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