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Method for hybrid BGA, SMT, and THM leads for solder-joint reliability enhancement

IP.com Disclosure Number: IPCOM000009094D
Publication Date: 2002-Aug-07
Document File: 9 page(s) / 321K

Publishing Venue

The IP.com Prior Art Database

Abstract

Disclosed is a method for hybrid ball grid array (BGA), surface-mount technology (SMT), and through-hole mount (THM) leads for solder-joint reliability enhancement. Benefits include improved reduction of defects.

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Method for hybrid BGA, SMT, and THM leads for solder-joint reliability enhancement

Disclosed is a method for hybrid ball grid array (BGA), surface-mount technology (SMT), and through-hole mount (THM) leads for solder-joint reliability enhancement. Benefits include improved reduction of defects.

Background

      Reflow soldering is fundamentally a heat transfer process that raises all solder joints on the surface-mount assembly (SMA) above the melting temperature of the solder alloy. The molten solder flows and forms the shape of the final joint. In the case of a BGA, the many joints (balls) must all be heated relatively uniformly to conform to the time-temperature requirements of the reflow specification, such as peak temperature and liquidus time.

      The conventional integrated heat spreader (IHS) solution (see Figures 1 and 2) maintains package substrate warpage within specification (<8 mils) on a large die. The IHS covers ~90% of the package and adds significant thermal mass to the total package. This increase has led to the difference in the reflow peak temperature between the solder joint at the package corner and center of the package. For example, this difference in flip-chip ball grid array 2 (FCBGA2) is 15ºC (see Figure 3). The specification for reflow peak temperature is from 205‑225ºC (20ºC tolerance). The temperature delta reduces process robustness through package dynamic warpage during reflow that leads to open and abnormally shaped solder joints with different standoffs (see Figures 4 and 5).

              The stress from the coefficient for thermal expansion (CTE) mismatch on a package with an IHS is shifted from the die shadow to the package corner.

              Conventionally, there is no practical solution to the issue of large temperature delta within a package (center of the package and package corner peak temperature).

      Some confusion exists about the mechanism of heating in convection-dominant ovens, indicating that heated gas flows under the package and heats the entire ball through convection. This theory is incorrect for two reasons. The first is the gas flow pattern in this style of oven, which is primarily vertical impingement uniformly across the entire area of the assembly. This pattern precludes the presence of pressure differentials that would be required to move the heated gas through the center of the package from one edge to the other.

      The second reason is that the geometry between the package and the PCB leaves a very narrow space obstructed by numerous closely spaced balls, all of which would constitute a high resistance to the movement of any significant volume of gas. Convection-dominant ovens provide excellent reflow of BGA components not because they force hot gas under the packages but, rather, because they supply a high level of heat transfer (to top and bottom surfaces) at relatively low gas temperatures, which prevents the overheating of smaller devices.

General description

              The disclosed method is hybrid BGA and SMT leads / THM leads...